Light
Light: Light is a form of energy. It
helps in the sense of vision. When light falls on an object, some of the light
is reflected back to our eyes. Thus, we are able to see an object because of
light being reflected from the object.
Reflection
of Light:
When light falls on a shiny surface,
most of it bounces back. This phenomenon is called reflection of light.
Laws
of Reflection:
There are two laws of reflection
which are as follows:
- First Law of Reflection: The incident ray, the reflecting ray and normal at the point of incidence; all lie in the same plane.
- Second Law of Reflection: Angle of incidence is always equal to the angle of reflection. In the given figure; the angle of incidence is shown by blue colour and the angle of reflection is shown by red colour.
Formation of Image in Plain Mirror
In this figure; PQ is a plain mirror
and O is an object. OA and OC are the incident rays; coming from the object.
The incident rays are reflected from the mirror and reflected rays emerge as AB
and CD. If AB and CD are extended behind the mirror, they appear to meet at
point ‘I’. This is the point where image is formed. If OM is object distance
and MI is image distance, then in plain mirror; OM = MI. This means image
distance is equal to object distance in case of a plain mirror.
Some features of image formed by a
plain mirror:
- Image is always formed behind the mirror.
- It is a virtual image; which means it cannot be obtained on screen.
- Image is upright and laterally inverted.
- Image size and distance are equal to object size and distance.
Regular Reflection:
When all the reflected rays are
parallel to each other; this case of reflection is called regular reflection.
Regular reflection happens from a smooth surface, e.g. a mirror. We get to see
clear images when image is formed by regular reflection.
Irregular
Reflection:
When the reflected rays are not
parallel to each other; this case of reflection is called irregular reflection.
Irregular reflection happens from a rough surface. The laws of reflection are
obeyed in this case as well but because of the irregularities in the surface,
the reflected rays are not parallel. We get to see somewhat blurred images when
image is formed by irregular reflection. The relative clarity of an image
depends on the relative smoothness of the reflecting surface.
Multiple
Images:
When two or more mirrors are placed
at some angles to each other, we get to see multiple images. Let us take an
example in which two mirrors are placed opposite to each other. If an object is
placed between them, its image is formed in both the mirrors. The image in one
mirror would act as an object for another mirror and this sequence would
continue. This will result in the formation of multiple images.
The number of images formed depends
on the angle between the two mirrors. This can be calculated by using the
following formula:
Number of Images = 
So, if the given mirrors are at a
right angle to each other, 3 images will be formed. If the given mirrors are at
30° angle, we shall get 11 images. When the mirrors are kept opposite and
parallel to each other, there would be infinite number of images formed.
Light
HUMAN EYE
Irish: Irish is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.
Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.
Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.
Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:
- Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
- Rod Cells: The rod cells are sensitive to dim light.
Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.
Persistence of Vision:
An image stays on the retina for about 1/16 of a second. This feature is called persistence of vision. Due to this, when many still images are shown in a sequence; they give the illusion of moving images. Movies and animation are made by exploiting this property of the human eye.Care of Eyes:
Eyes are very important organ and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:- Do not read in too bright or too dim light.
- Do not look directly at a bright object or at the sun.
- Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
- If something gets into the eye, do not rub the eye. Wash it with cold water.
- In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.
Vision Defects:
Some of the vision defects are as follows:Myopia: A person suffering from myopia finds it difficult to see a distant object. This disease is also called short sightedness or near sightedness. Spectacles with suitable lenses can improve the vision in a myopic person.
Hypermetropia: A person suffering from hypermetropia finds it difficult to see a nearby object. This disease is also called long sightedness. Spectacles with suitable lenses can improve the vision in a hypermetropic person.
Cataract: The cornea becomes cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cornea is cleaned and an artificial lens is transplanted in the eye.
Visually Challenged Person:
Some people face with disability of vision. This disability can be partial or complete. Such persons are called visually challenged persons. For a visually challenged person; life can be very difficult. These people usually show a marked development of other senses; like the sense of hearing and sense of touch. Many aids have been devised to make their life easy. They can be divided into two categories, viz. optical and non-optical aids.Optical Aids: Optical aids can help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. TV monitors, magnifying devices and telescopic devices come under this category.
Non-optical Aids: Non-optical aids are helpful for a person who is completely visually challenged. These aids rely on the senses of hearing and touch. Aids which rely on the sense of touch are called tactual aids. Tactile buttons on the pedestrian light and in public transport are examples of tactual aids. Tactile strips at the edge of the platforms are also meant for visually challenged persons. Even the currency notes have tactile markings so that a visually challenged person can recognize notes of different denominations.
In some countries; specially trained guide dogs are pressed into the service of a visually challenged person.
NCERT
Solution (part 2)
Question – 9 - Describe the
construction of a kaleidoscope.
Answer: A kaleidoscope is composed
of a hollow tube. Three mirrors are placed in the form of a triangular tube and
their reflecting surfaces face each other. On end of the tube is covered with a
transparent sheet and another end is covered with an opaque sheet. There is an
eyehole in the opaque sheet. Bits of glass are filled inside the tube. When a
kaleidoscope is turned, we get to see various patterns in it. These patterns
are formed because of multiple reflection.
Question – 10 – Draw a labeled
sketch of the human eye.
Answer
Question – 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 teachers advise?
Answer: Laser rays can be very
harmful for the eyes. Hence, a laser torch should never be used for activities
related to light.
Question – 12 - Explain how you can
take care of your eyes.
Answer: Some tips for care of eyes
are as follows:
- Do not read in too bright or too dim light.
- Do not look directly at a bright object or at the sun.
- Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
- If something gets into the eye, do not rub the eye. Wash it with cold water.
- In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.
Question – 13 - What is the angle of
incidence of a ray if the reflected ray is at an angle of 90° to the incident
ray?
Answer: We know that the angle of
incidence is equal to the angle of reflection.
Given: ∠ i + ∠ r = 90°
Since ∠
i = ∠ r hence;
∠ i + ∠ i = 90°
Or, 2∠
i = 90°
Or, ∠
i = 45°
Question – 14 - How many images of a
candle will be formed if it is placed between two parallel plane mirrors
separated by 40 cm?
Answer: When the mirrors are
parallel to each other, then we get infinite number of images.
Question – 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.
Answer: 
Question – 16 - 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?
Answer: Boojho cannot see his image
in the mirror. But he can see P, Q and R. This can be shown by the following
figure.
Question – 17 :
- Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
- Can Paheli at B see this image?
- Can Boojho at C see this image?
- When Paheli moves from B to C, where does the image of A move?
-
Answer: Both Paheli (at B) and Boojho
(at C) can see the image. The position of the image will not change with change
in position of the viewer, because the object remains are the same position.
Image position can be drawn as in the previous question.
Question – 1 - Suppose you are in a
dark room. Can you see objects in the room? Can you see objects outside the
room? Explain.
Answer: It is not possible to see in
a dark room because of absence of light. We know that we are able to see
something when light falls on it. If there is light outside the room, then the
objects outside the room can be seen.
Question – 2 - Differentiate between
regular and diffused reflection. Does diffused reflection mean the failure of
the laws of reflection?
Answer: Regular reflection happens
from a highly smooth surface, while irregular reflection happens from a rough
surface. Clear images are formed in case of regular reflection, while blurred
images are formed in case of irregular reflection.
The laws of reflection are followed
in every situation. Hence, diffused reflection does not mean the failure of the
laws of reflection.
Question – 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.
- Polished wooden table
- Chalk powder
- Cardboard surface
- Marble floor with water spread over it
- Mirror
- Piece of paper
Answer: Regular reflection will only
take place in case of a mirror. In other cases, irregular reflection will take
place; because the surfaces are not smooth.
Question – 4 - State the laws of
reflection.
Answer: First Law of Reflection: The
incident ray, the reflecting ray and normal at the point of incidence; all lie
in the same plane.
Second Law of Reflection: Angle of
incidence is always equal to the angle of reflection. In the given figure; the
angle of incidence is shown by blue colour and the angle of reflection is shown
by red colour.
Question – 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.
Answer: For this, take a chart
paper, a torch and a plain mirror. Paste a black paper on the front of the
torch and make a small hole at the centre of black cover. This will give you a
thin beam of light.
Keep the chart paper on a table in a
way that some portion of the chart paper is outside the edge of the table. Keep
the mirror vertically on the chart paper.
Direct the beam of light on the
mirror so that the incident ray is along the surface of the chart paper. Mark
the incident ray, the normal and the reflected ray.
After that, fold the portion of the
chart paper which is jutting out of the table. Repeat the above step and
observe the incident ray and reflected ray.
It is observed that the reflected
ray cannot be seen on the folded portion of the chart paper. The folded portion
of the chart paper is in a different plane than the remaining part of the chart
paper. This proves that the incident ray, the normal and the reflected ray; all
lie in the same plane.
Question – 6 - Fill in the blanks in
the following :
(a) A person 1 m in front of a plane
mirror seems to be _______________ m from his image.
Answer: 2 m
(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 ____________
Answer: left, left
(c) The size of the pupil becomes
____________ when you see in dim light.
Answer: more
(d) Night birds have ____________
cones than rods in their eyes..
Answer: less number of
Question – 7 - Angle of incidence is
equal to the angle of reflection
- Always
- Sometimes
- Under special conditions
- Never
Answer: (a) Always
Question – 8 - Image formed by a
plane mirror is
- virtual, behind the mirror and enlarged
- virtual, behind the mirror and of the same size as the object
- real at the surface of the mirror and enlarged
- real, behind the mirror and of the same size as the object.
Answer: (b) Virtual, behind the
mirror and of the same size as the object
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