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Important Question:-
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Read Also: Light-Reflection and Refraction Revision Notes
Multiple Choice Questions [MCQs]
Ques: When we see the stars in the night they appear slightly shifted from their actual position. Which phenomenon of light is responsible for that:
(i) Diffraction of light
(ii) Scattering of light
(iii) Refraction of light
(iv) Reflection of light
Answer: (iii) Refraction of light
Ques: Due to which phenomenon of light the colour of the sky looks blue:
(i) Reflection of light
(ii) Refraction of light
(iii) Dispersion of light
(iv) Scattering of light
Answer: (iv) Scattering of light
Ques: When a ray or a beam of white light falls on a glass prism, which colour of light will deviate least is:
(i) Violet
(ii) Red
(c) Green
(d) Blue
Answer: (b) Red
Ques: The muscular part of the eye or diaphragm that helps the pupil to control its size is:
(i) cornea
(ii) ciliary muscles
(iii) iris
(iv) retina
Answer: (iii) Iris
Ques: What are the properties of image formed at the retina of the human eye:
(a) virtual and inverted
(b) real and inverted
(c) real and erect
(d) virtual and erect
Answer: (ii) Lens of the eye is convex so the image formed is real and inverted
The video below explains this:
Human Eye One Shot Detailed Video Explanation:
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Short Answer Type questions
Ques: What do you understand about the Tyndall effect? State with an example of a condition in which the Tyndall effect can be observed.
Answer: It is the phenomenon in which the scattering of light by the colloidal particles takes place and is called the Tyndall effect. When a ray of light enters a dark room and its dust particles are visible. It is due to the scattering of light or the Tyndall effect.
Ques: What is the main objective of a pupil?
Answer: The main function of the pupil is to control and regulate the amount of light entering the eyes.
Ques: What is the role of ciliary muscle present in human eyes?
Answer: Ciliary muscles help the eyes to increase or decrease the curvature of the lens present in the eyes. These ciliary muscles help the retina in the eyes to focus the image of an object.
Ques: What is the type of lens present in human eyes? State the properties of an image formed by an eye lens?
Answer: In the human eye, the image is formed at the retina present in the human eye. The lens present in the human eye is convex so a real and inverted image is formed.
Ques: State the role of the cornea and lens of the eye?
Answer: The cornea is the outer layer of an eye which is a clear, round structure that covers and protects the iris and the pupil. The cornea controls and regulates light rays into the eye which gives a retina sharp & clear vision. The lens is located behind the iris and is normally clear.
Ques: At night the sky appears black instead of blue to an astronaut. What is the cause for that state?
Answer: In space, there is no atmosphere and air so the white light from the sun cannot scatter into different colours. As the atmosphere and air are not present in space and scattering of light cannot take place the different colours of light did not reach the eyes of an astronaut which is why the sky appears black or dark instead of blue to an astronaut in space.
Long Answer Type Questions
Ques: To an astronaut in space, the sky appears black instead of blue at night? What is the cause for that state?
Answer: In space, there is no atmosphere and air so the white light from the sun cannot scatter into different colours. As the atmosphere and air are not present in space and scattering of light cannot take place the different colours of light did not reach the eyes of an astronaut which is why the sky appears to be dark black but not blue to an astronaut in space.
Ques: What is the phenomenon of the spectrum? On passing through a glass prism why the lights of different colours deviate at different angles?
Answer: The white light consists of 7 colours. The ray beam or band of this white light is called the Spectrum of white light. When this spectrum passes through a glass prism scattering of light takes place and the white light scatters in different colours.This band of light formed with seven different colours. The colour sequence of this spectrum is given by the short term
V I B G Y O R —
- V - Violet
- I - Indigo
- B - Blue
- G - Green
- Y - Yellow
- O - Orange
- R - Red
The speed of light is different for different mediums like (glass, water) is different for different colours. Because the differences in the speeds for different colours lead to the differences in refractive index for different colours. Also, the different colours of light have different wavelengths, the wavelength for violet is the least and for red it is the highest. As it is discovered that the wavelength for red is the highest while for violet is the lowest. On passing through the glass prism all the colours in white light due to different wavelengths and refractive index emerge from the prism at different angles. The angle of deviation is maximum for Violet and minimum for red.
Ques: With the help of a labelled diagram explain the cause of the twinkling of a star?
Or What is the cause for the Twinkling of the star? Explain with a labelled diagram.
Answer: The main reason for the twinkling of the star is the refraction of the starlight. The phenomenon of refraction and reflection takes place when light travels from a rarer medium to a denser medium. This is due to the change in the refractive index. The atmosphere acts as a denser medium so the light rays coming from the star bend toward the normal, so we see a slight difference in the actual position and apparent position of the star. Because of the atmosphere present and the physical conditions of the earth's atmosphere, the apparent position of the star is not fixed but keeps on changing. Since the stars are too far away from us they are visible as a point of the light source. In the path the rays of light coming from the stars go on several slight differences, the real position of a star fluctuates and the apparent position looks like it is changing. The amount of light entering our eyes flicks the stars which are why sometimes the stars appear bright and sometimes it faints which makes it appear dimmed. This is the cause of the twinkling of a star.
Ques: What is called the scattering of light? Also, explain the reason for the following terms:
Answer: Scattering of light is the phenomenon or process in which a beam or ray light throws light in various and random directions. Scattering of the light takes place when it falls on various types of surfaces or surrounding particles in the path of light. The beam of light scatters at different angles and the colour of light and angle of deviation depends on the size of the scattering particles in the atmosphere. The larger particles present in the atmosphere scatter most of the light to a longer wavelength which is why the scattered light appears white. The smallest or very minute particle scatters the least. Such particles are mainly present to appear blue in the atmosphere. This might have the least wavelength and deviates the least. The colour of the sky appears to be blue because of the scattering of white light. As we know the molecules of air and other fine particles in the atmosphere have a very less wavelength and are scattered the most as compared to other colours present in visible light. The wavelength of red light is the least and the wavelength of blue light is the most. These particles scatter most of the light rays with a shorter wavelength and blue or violet has the shortest wavelengths. This scattering of light is more for blue light so the light which enters our eyes makes the sky appear blue. Also, the red light has the maximum wavelength but it scatters the least so when we see around the sun or close to it appears to be red.
Therefore at the time of sunrise, the sky looks blue and the the sun seems to be reddish due to the scattering of white light.
Ques: What are the three common refractive defects of vision? State the ways how to correct these defects.
Answer: More commonly there are three types of defects in vision.
They are generally known:
- Myopia
- Hypermetropia
- Presbyopia
- Myopia: Myopia is the defect or condition of the eye vision due to which a person can see the objects near clearly, but cannot see the far objects. It is also known as short-sightedness.
- Causes of myopia: Myopia is caused because the eyeball becomes large, this leads to a decrease in the focal length of the eye lens due to which it becomes more convergent.
- Correction: Myopia can be treated or corrected by placing a concave lens of suitable focal length in the spectacles of such a person.
- Hypermetropia: It is the condition or defect in vision in which the person can see the far objects clearly but cannot see near objects. It is also known as Long-sightedness.
- Causes of hypermetropia: Sometimes there is a normal increase in focal length of the eye lens which makes it less convergent. Also due to the shortening of the eyeball.
- Correction: It is corrected by placing a convex lens of suitable power in the spectacles or wearing an artificial convex lens for the eyes.
- Presbyopia: It is the condition when the power of the lens in the eyes decreases. It happens with the ageing of a person. It occurs because of the gradual weakening of the ciliary muscles and decreasing flexibility of the eye lens. This defect is called presbyopia.
- Causes of presbyopia: The main cause of this defect is the weakening of the ciliary muscles of the eyes. A person with this defect either suffers from myopia or hypermetropia.
- Correction: This defect or condition is corrected and treated by placing bifocal lenses of suitable focal lengths in the spectacles. In such cases, the upper part of the spectacles is made using a concave lens for correcting myopia to see distant objects clearly, while at the lower part of the lens convex lens is used to correct the hypermetropia to see the nearby objects.
Ques: Give the answers for the following parts:
(a) A student has some sort of defect in his vision and he is not able to see distant objects placed beyond 5 m. What is the defect called and list some possible reasons why it happens and explain with the help of a ray diagram?
(i) Why is the student not able to see the objects placed beyond 5 m from his eyes.
(ii) By which lens the condition can be corrected and how is this defect corrected by the use of this lens?
(b) If in this case, the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.
Answer: (a) The condition or defect that arises in the eyes of a student is called Myopia. There are two possible ways for the condition of defect arouse in a vision of student;
Maybe the size of the eyeball becomes larger and more convergent. The lens of the eye becomes thicker than its normal thickness during its power of accommodation.
(i) Condition before placing a corrective lens.
(ii) Myopia can be corrected or treated by placing a concave lens. The labelled diagram after placing corrective lens is given below:
(b) If focal length is = - 5m (because the lens is concave)
- Power of lens = ? (Let power = P)
- Formula to find power of lens is :
- P = 1 / f
- P = 1 / – 5
- P = – 0.2 Diopter
- Hence the power of the correcting lens having a focal length of 5m is – 0.2 D and the lens is concave.
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