NOTES
What makes things visible?
We can see the world through our eyes or our sense of sight. However, we cannot see anything without the light, that is, in darkness. When light falls upon or is emitted by an object our eyes detect that light and hence the object becomes visible to us.
he laws of reflection
What is the reflection of light?
Figure 3 Reflection of Light
When light strikes a surface and bounces back, it is called reflection.
Incident ray - The ray of light that falls on the surface is called incident ray.
Reflected ray - The ray of the light that gets reflected back from the surface is called reflected ray.
Normal - It is an imaginary line that is drawn perpendicular to the reflecting surface at the point where the incident ray strikes the surface or at the point of incidence.
The angle of incidence (θi) - It is the angle that the incident ray makes with the normal.
The angle of reflection (θr) - It is the angle that the reflected ray makes with the normal.
For an incident ray, there is always one reflected ray.
First Law of Reflection
The angle of incidence and the angle of reflection are always equal to each other. In other words, the angle by which a ray of light is incident on a surface will always be equal to the angle by which it is reflected back from the surface.
The Second Law of Reflection
The incident ray, the reflected ray and normal all lie in the same plane.
What is lateral inversion?
When an image is formed in a mirror the, the left side of the object appears at the right side of the image and vice versa. This is called a lateral inversion.
Regular and Diffused reflection
| Regular Reflection | Diffused Reflection |
When parallel rays of light fall on a smooth surface, they are reflected back as parallel rays only. This is called regular reflection. | When parallel rays of light fall upon an irregular surface, they are reflected back into all the directions. This is called diffused reflection. |
It is also called specular reflection. | It is also called irregular reflection. |
It leads to the formation of an image. | It enables us to view different objects present in our environment. |
For example, the image formed by a plane mirror. | For example, the reflection of rays of light through a wall |
The white light consists of seven colours namely VIBGYOR (violet, indigo, blue, green, yellow, orange and red).
This arrangement of seven colours of a white light is called as Spectrum.
All these colours mixed together and form the white light. However, due to the reflection of light, the seven colours can split and be seen as separate colours rather than a single white light.
This phenomenon of the splitting of light into its constituents is called Dispersion of light.
The structure and working of the human eye
Eyes are sensory organs that allow us to see things present in the environment.
The eyes absorb the light rays and form a visual image and transform this information of the image to the brain.
The eye has a spherical structure which is called the eyeballs. The eyeballs are attached in the eye socket and various muscles are connected to them called the eye muscles. These are muscles allow the movement of the eyeballs.
The eyes are protected with eyelids. They prevent an excess of light and dust to enter into the eyes and therefore save the eyes from any outer objects.
Figure 13 Human Eye
The parts of the human eye
Cornea - It is a transparent covering present on the outer side of the eyes. It consists of 6 layers. Tear fluid covers cornea and protects the eye.
Iris - It is a dark coloured muscular structure present in the centre of the cornea. The Iris consists of a small opening call Pupil through which the light enters into the eye. The Iris has colourful pigments like grey, blue, black, green, brown etc. The amount of light that enters the eye is controlled by the Iris. In other words, Iris controls the Pupil.
Lens - It is like a sphere of liquid present inside the eye. The eye lens is located behind the Pupil which helps in the formation of an image in the eye.
Retina- Retina is a layer located behind the lens at the back of the eye. The eye lens focuses the light that enters into the eye over the retina. The retina consists of different Nerve cells that perform different tasks. There are sensory cells that detect the light. They then transform that information into an electrical impulse. These electrical impulses are sent to the brain via the optic nerve. Two types of vision cells in the retina are:
Rods or chopsticks - They get activated in darkness or dim-light and the responsible for light-dark vision.
Cones - they react to bright light and therefore sense colour in an object.
Vitreous Chamber - The inner space of the eyeball called the transparent vitreous chamber consists of a gel-like substance. This substance maintains the right pressure in the eye and maintains the stability of the eyeball.
Blind spot - There is a point in the eye located at the junction of the retina and the optic nerve where no sensory cells are present. This spot is therefore called the blind spot as it does not support any vision.
Persistence of image on the retina
The image that is formed on the retina persists for 1/16th of a second.
Therefore, if one tries to move 16 still images per second of a moving object in front of our eye it appears as if the object is moving. This is how animation films and movies work. They are a collection of separate pictures which are moved in a sequence.
However, this movement is so fast, around 24 pictures in a second, that it appears as if they are moving.
The vision may decrease with age
Normally a person can see distant and nearby objects through the eyes.
A person with normal eyes can read most comfortable at a distance of 25cm. However, this distance can decrease with age.
Also sometimes people can see far objects clearly but not the objects that are nearby (farsightedness or hyperopia).
Similarly, some people can see the nearby objects clearly but not the far objects (nearsightedness or myopia). Hence they use suitable lenses to correct such defects.
Sometimes with old age, the lens of the eye can become cloudy or foggy. This condition is called cataract with results in loss of vision.
However, this defect can be rectified by removing the old lens from the eye and inserting an artificial lens at that place.
| Defect in Eye | Causes |
1. Presbyopia | solidification of lens fluid |
2. Cataract | protein structures of lens clubbed together |
3. Glaucoma | abnormal pressure inside the eye |
4. Myopia | due to a strong refractive index of the eye |
5. Hyperopia | due to a weak refractive index of the eye |
6. Astigmatism | non-uniform curvature of the cornea |
How to take care of the eyes?
One should go for a regular eye check up.
Wear spectacles if the eyesight is found weak.
Always study or watch TV in a sufficient light. Excess or insufficient light both can lead to problems. Insufficient light can cause strain and leads to a headache.
Do not look at the sharp source of light like the sun directly. Excess of light can damage the eyes.
If dust enters into the eye one should not rub them and rather wash them straightway.
Maintain a normal distance while reading or watching TV.
Take a balanced diet to ensure that you get the right nutrients.
What is night blindness?
Night blindness also called nyctalopia is a defect in the vision in which people cannot view things properly in dim light or at night. It mainly occurs due to the lack of vitamin A in our body. Hence we should always take foods that are rich in vitamin A such as broccoli, green vegetables, carrot, milk, eggs, curd, papaya, mango etc.
How can visually impaired people read and write?
Visual impairment is also known as vision loss.
A person with vision loss cannot see at all.
This loss of vision is up to a degree that cannot be corrected using usual means like using glasses or lenses.
The people who have no eyesight at all develop the other senses of listening and touching sharply which allows them to do their day to day activities.
However, there are certain resources that have been developed to help such people:
| Non-Optical Aids | |
Visual Aids | They are used to magnify words ensuring appropriate intensity of light falls upon the eyes so that the person can recognize the word. |
Tactual Aids (using sense of touch) | They include Braille writer Slate and Stylus which allow a person to read and write. |
Auditory Aids (using sense of listening) | They include devices like audio tapes, talking books etc |
Electronic Aids | They include talking calculators and computers, closed circuit televisions, audio CDs, that can help visually challenged people in listening and writing. |
| Optical Aids | |
Bifocal Lenses | They all are used to rectify the limitations of the eyes. |
Contact Lenses | |
Tinted Lenses | |
Magnifiers | |
Telescopic Aids | They are used to view the chalkboard and class demonstrations. |
Figure 14 Magnifier Optical Aid for Visually impaired
The Braille system
Braille is a tactual aid for visually challenged people that allow them to read and write.
It was developed by a visually challenged person called Louis Braille. This system was published by him in 1821 however the present Braille System was adopted in 1932.
For many common languages including Hindi, Sanskrit, Tamil, Telugu, Mathematics and scientific notations a Braille code is present.
In the Braille code, there are 63 Dots or 63 Characters.
These are arranged in a cell of two vertical rows having three dots each.
The patterns are embossed on a Braille Sheet that allows a person to read by touching the pattern. Each character in the Braille system can represent:
a letter
a combination of letters or a word
a grammatical sign
Figure 15 The Braille System for English Alphabets
Eyes of animals have different shapes
Crab: Crabs have small eyes but they can look all around with them even behind them.
Butterflies: Butterflies have big eyes that consist of thousands of small eyes which allow them to see in the front and at the back.
Night owls (Night Birds): Night owls are able to see things more clearly in the night rather than day time. This is so because the cornea of their eyes is large with a large pupil which allows light to enter into the eye at night time. However, their eyes have a large number of rod cells rather than cones that allow them to see the things at night easily.
Kites and Eagles (Day Birds): These birds can see very well during daytime but have decreased vision at night. This is because they have a large number of cone cells and a few rod cells.
Exercise Questions
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Soln:
If a person is inside the room where there is no light, it is then impossible to visualize the object inside the room but the object out of the room can be seen easily.
When light falls on eyes after reflecting from the object, it becomes visible. If the room is dark, then the object which is in the room reflects no light. Hence, the person is not able to see the objects in the room where there is no light.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Soln:
S.No Regular Reflection Diffused Reflection 1. It occurs when the surface is smooth. It occurs when the surface is a rough surface. 2. Reflected rays move in a particular direction. Reflected rays scattered in random directions. Example: Reflection by the plane mirror Example: Reflection by the road surface. The laws of reflection have not failed because each ray obeys the law of reflection. All the reflected rays are parallel to each other, whereas in diffused reflection the rays aren’t parallel to the incident rays.
3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper
Soln:
a) The wooden table that has been a polished-Regular reflection
The surface that has been recently polished can be a good example of a smooth surface. The wooden table that has been polished has a surface that is smooth.
b) White Chalk powder that is used in school- Diffused reflection
Chalk powder spread on a surface is an example of an irregular surface. Hence, it is rough. Therefore, the diffused reflection will appear from chalk powder.
c) Cardboard surface- Diffused reflection
The surface of the cardboard is a kind of irregular surface. Hence, the diffused reflection will take place from a cardboard surface.
d) Marble floor – Regular reflection
Marble floor can be a good example of a surface that is regular. Since water makes the ceramic glossy, thus, the reflections that are regular occur on this surface.
e) Mirror- Regular reflection
A mirror has a very smooth surface hence it gives a regular reflection.
f) Piece of paper- Diffused reflection
Although a piece of paper may look smooth, it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.
4. State the laws of reflection.
Soln:
The law of reflection states that
a) The angle of reflection and the angle of incidence both are always equal to one another.
b) The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all come on the same plane.
5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Soln:
On a table, place a plane mirror perpendicular to the plane of the table. Make a small hole in a paper and hold it perpendicular to the plane of the table. Try to do this experiment in a dark room. Take one more piece of paper and place it on the table so that it makes contact with the mirror. Draw a line perpendicular to the mirror on the piece of paper which is on the table. Now beam light rays with the help of a torch through the small hole such that the beam of light hits the normal at the bottom of the mirror. The ray of light will be reflected in the light rays from the hole are incident on the mirror. Looking at the piece of paper on the table, we can easily show that the incident ray, the normal line and the reflected ray at the point of incidence lie in the same plane.
6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image.
(b) If you touch your ____________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________.
(c) The size of the pupil becomes ____________ when you see in dim light.
(d) Night birds have ____________ cones than rods in their eyes.
Soln:
(a) A person 1 m in front of a plane mirror seems to be 2m away from his image.
(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with the left hand.
(c) The size of the pupil becomes large when you see in dim light.
(d) Night birds have fewer cones than rods in their eyes.
Choose the correct option in Questions 7 – 8
7. The angle of incidence is equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Soln:
Answer is (a) Always
8. Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
Soln:
Answer is (b) virtual, behind the mirror and of the same size as the object.
9. Describe the construction of a kaleidoscope.
Soln:
The construction of a kaleidoscope:
Take three rectangular mirror strips of dimensions 15cm x 4cm (l x b) and join them together to form a prism. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism. This circular tube is now closed at one end with a cardboard disc. This disc has a hole through which we can see. At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move.
10. Draw a labelled sketch of the human eye.
Soln:
11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Soln:
Her teacher advised her not to do so because of the intensity of the laser light is very high, it is harmful to the human eyes. It can cause damage to the retina and leads to blindness. Hence, it is advisable not to look at a laser beam directly.
12. Explain how you can take care of your eyes.
Soln:
The following points help a person to take care of his eyes:
a) Reading should not be done in bright light as well as in dim light.
b) He should visit an eye specialist on a regular interval of time.
c) If any small insects or dust particles enters his eyes, do not rub them but clean them immediately with cold water.
d) He should avoid direct exposure of sunlight to the eye.
e) While reading, there should be a distance of at least 25 cm between the eyes and the book.
13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Soln:
If the reflected ray is at the angle of 90o to the incident ray, then the angle of incidence is 45o. According to the law of reflection, the angle of incidence and the angle of reflection are equal. Therefore, the angle of incidence and the angle of reflection both are 90/2=45o.
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Soln:
If a candle is placed between two parallel plane mirror separated by 40 cm, then the multiple and infinite images will be formed due to the multiple reflections between the mirrors. The infinite numbers of images are formed when two mirrors are placed parallel to each other.
15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.
Soln:
The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle of 60°.
The incident ray OA reflects at point O
Since Angle of Reflection = Angle of Incidence
OO’ makes an angle 300 with normal of first mirror
Now, drawing normal at O’
The two normals intersect at 900 angle
And applying angle sum property in △ OXO’
We get
Angle of Incidence in 2nd mirror = 600
Applying law of reflection in 2nd mirror,
We get the below figure
6. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q and R?
Soln:
Boojho cannot see his image because the reflected ray won’t reach his eyes.He can see the image of objects situated at P, Q because the rays coming from P and Q get reflected by the mirror and reach his eyes. Boojho can’t see the image of objects situated at R because the ray from object R does not get reflected.
17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?
Soln:
a) Image of an object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror
b)Yes Paheli at B can see this image.
c) Yes Boojho at C can see this image.
d) Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.
