A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
+ 0.5 D
- 0.5 D
+ 0.2 D
- 0.2 D
A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
The near point of his eyes has receded away
The near point of his eyes has come closer to him
The far point of his eyes has come closer to him
The far point of his eyes has receded away
A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in Figure 11.1. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
(i)
(ii)
(iii)
(iv)
At noon the sun appears white as
light is least scattered
all the colours of the white light are scattered away
blue colour is scattered the most
red colour is scattered the most
Which of the following phenomena of light are involved in the formation of a rainbow?
Reflection, refraction and dispersion
Refraction, dispersion and total internal reflection
Refraction, dispersion and internal reflection
Dispersion, scattering and total internal reflection
Twinkling of stars is due to atmospheric
dispersion of light by water droplets
refraction of light by different layers of varying refractive indices
scattering of light by dust particles
internal reflection of light by clouds
The clear sky appears blue because
blue light gets absorbed in the atmosphere
ultraviolet radiations are absorbed in the atmosphere
violet and blue lights get scattered more than lights of all other colours by the atmosphere
light of all other colours is scattered more than the violet and blue colour lights by the atmosphere
Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
Red light moves fastest
Blue light moves faster than green light
All the colours of the white light move with the same speed
Yellow light moves with the mean speed as that of the red and the violet light
The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light
is scattered the most by smoke or fog
is scattered the least by smoke or fog
is absorbed the most by smoke or fog
moves fastest in air
Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
Dispersion of light
Scattering of light
Total internal reflection of light
Reflection of light from the earth
The bluish colour of water in deep sea is due to
the presence of algae and other plants found in water
reflection of sky in water
scattering of light
absorption of light by the sea
When light rays enter the eye, most of the refraction occurs at the
crystalline lens
outer surface of the cornea
iris
pupil
The focal length of the eye lens increases when eye muscles
are relaxed and lens becomes thinner
contract and lens becomes thicker
are relaxed and lens becomes thicker
contract and lens becomes thinner
Which of the following statement is correct?
A person with myopia can see distant objects clearly
A person with hypermetropia can see nearby objects clearly
A person with myopia can see nearby objects clearly
A person with hypermetropia cannot see distant objects clearly
Draw ray diagrams each showing (i) myopic eye and (ii) hypermetropic eye.
Ray diagrams show:
(i) Myopic eye: Light rays from a distant object focus in front of the retina.
(ii) Hypermetropic eye: Light rays from a nearby object focus behind the retina.
A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give her? Draw ray diagram for the correction of this defect.
The student is suffering from myopia (near-sightedness). A doctor advises her to use a concave lens of appropriate power to correct the defect.
A concave lens diverges the incoming rays so that they focus on the retina.
How are we able to see nearby and also the distant objects clearly?
The human eye is able to see objects at different distances clearly by changing the focal length of the eye lens using its power of accommodation.
A person needs a lens of power −4.5 D for correction of her vision. (a) What kind of defect in vision is she suffering from? (b) What is the focal length of the corrective lens? (c) What is the nature of the corrective lens?
(a) The person is suffering from myopia.
(b) The focal length \( f = \dfrac{1}{P} = \dfrac{1}{-4.5} = -0.22 \text{ m} \).
(c) The corrective lens is a concave lens.
How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.
Two identical prisms are arranged such that one is placed inverted with respect to the other. The first prism disperses white light into its spectrum, and the second prism recombines the spectrum to form white light again.
Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also indicate the order of the colours of the spectrum obtained.
When white light is incident on a prism, the emergent beam splits into a spectrum of seven colours. The order from top to bottom is: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Is the position of a star as seen by us its true position? Justify your answer.
No. Due to atmospheric refraction, the light from stars bends as it enters layers of air with varying refractive indices. Hence, the star appears slightly higher than its actual position.
Why do we see a rainbow in the sky only after rainfall?
After rainfall, a large number of water droplets remain suspended in the atmosphere. These droplets act like small prisms and disperse sunlight, producing a rainbow.
Why is the colour of the clear sky blue?
The blue colour of the sky is due to the scattering of light. Blue light is scattered the most by atmospheric particles because it has a shorter wavelength.
What is the difference in colours of the Sun observed during sunrise/sunset and at noon? Give explanation for each.
During sunrise and sunset, the Sun appears reddish because sunlight travels a longer atmospheric path and blue light is scattered away. At noon, the Sun appears white because it travels a shorter path and scattering is minimal.
Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
Structure of the Human Eye:
The human eye is a spherical organ approximately 2.5 cm in diameter. Its major parts are:
Functioning of the Eye:
Light entering the eye passes through the cornea, aqueous humour, pupil, lens, and vitreous humour. The lens focuses the light rays on the retina, where an image is formed. The optic nerve transmits the information to the brain, which interprets it as an upright image.
Seeing Nearby and Distant Objects—Accommodation:
The ability of the eye lens to change its focal length so that both near and far objects can be seen clearly is called accommodation.
Thus, the human eye adjusts its focal length to clearly see objects at various distances.
When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected.
Myopia (Near-sightedness):
A person is said to be myopic when he can see near objects clearly but cannot see distant objects distinctly. This happens because either the eye lens becomes too curved or the eyeball becomes elongated, causing the image of distant objects to form in front of the retina.
Correction: Myopia is corrected using a concave lens of suitable power. The concave lens diverges incoming light rays so that the image shifts back onto the retina.
Hypermetropia (Far-sightedness):
A person is hypermetropic when he can see distant objects clearly but cannot see nearby objects distinctly. This happens when the eyeball is too short or the eye lens becomes less curved, causing the image of nearby objects to form behind the retina.
Correction: Hypermetropia is corrected using a convex lens of suitable power. The convex lens converges the light rays before they enter the eye so that the image is formed on the retina.
Ray Diagrams:
Diagrams include:
Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Refraction Through a Glass Prism:
When a narrow beam of white light enters a triangular glass prism, it bends towards the normal at the first refracting surface due to its higher optical density. Inside the prism, the light travels towards the second refracting surface, where it bends again but away from the normal as it emerges into air.
Ray Path Description:
Angle of Deviation (\( D \)):
The angle between the direction of the incident ray and the emergent ray is called the angle of deviation. It is given by:
\[ D = (i + e) - A \]
where \( A \) is the angle of the prism.
How can we explain the reddish appearance of the Sun at sunrise or sunset? Why does it not appear red at noon?
Reddish Appearance at Sunrise and Sunset:
During sunrise and sunset, the Sun is near the horizon. The sunlight must travel a much longer distance through the Earth's atmosphere. Due to this long path, most of the shorter wavelengths such as blue and violet are scattered away by atmospheric particles.
The longer wavelengths like red and orange scatter least, so they reach the observer. Hence, the Sun appears reddish.
Why the Sun Does Not Appear Red at Noon:
At noon, the Sun is overhead and sunlight travels a much shorter distance through the atmosphere. Little scattering occurs, and all colours reach the observer almost equally. Thus, the Sun appears white or yellowish rather than red.
Explain the phenomenon of dispersion of white light through a glass prism, using a suitable ray diagram.
Dispersion of Light:
Dispersion is the phenomenon of splitting of white light into its constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a prism.
Cause of Dispersion:
Different colours of light have different wavelengths and therefore different refractive indices for the prism material. Violet light deviates the most, while red deviates the least.
Process:
Ray Diagram:
Diagram shows a beam of white light entering a prism, emerging separated into VIBGYOR colours.
How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Atmospheric Refraction:
Atmospheric refraction occurs because the Earth's atmosphere consists of layers of air with varying densities and refractive indices. Light rays from celestial objects bend gradually as they pass through these layers of different optical densities.
Twinkling of Stars:
Stars appear as point sources of light. Due to atmospheric turbulence, the refractive index of the layers keeps changing. This causes the path and intensity of star light to vary continuously, making the star appear to twinkle.
Why Planets Do Not Twinkle:
Planets appear larger because they are much closer to Earth. They act as extended sources of light. The variations in brightness due to atmospheric refraction average out over their larger apparent size, so planets do not twinkle.