1
A person is suffering from myopia. Draw a ray diagram showing how this defect can be corrected using a lens. Explain the role of the lens used for correction. 3marks
2
Explain the phenomenon of dispersion of white light through a glass prism. Why does white light split into different colours? 3marks
3
(a) What is the cause of the scattering of light in the atmosphere? (b) Explain why the sky appears blue during the day. 3marks
4
Why are danger signals usually red? Explain with reference to the scattering of light. 2marks
5
A rainbow is formed in the sky after a rain shower. Explain the formation of a rainbow with the help of a ray diagram, showing the processes involved. 5marks
6
Explain the formation of a rainbow using a ray diagram. Identify the phenomena involved. 3marks
7
(a) What is meant by the least distance of distinct vision? (b) Why does a person with myopia have to wear spectacles with concave lenses? 3marks
8
(a) What is meant by the power of accommodation of the eye? (b) How does it help in seeing objects at different distances? 3marks
9
(a) State the difference between myopia and hypermetropia. (b) How can these defects be corrected? 3marks
10
The sun appears reddish during sunrise and sunset. Explain this phenomenon. 2marks
11
Why do stars twinkle but planets do not? 3marks
12
A person can see objects placed within 15 cm from his eye clearly, but beyond 25 cm, he cannot see them distinctly.(a) Which optical defect is this person suffering from?(b) What is the cause of this defect?(c) Which type of lens is required to correct this defect? 3marks
13
Draw a labelled diagram of the human eye. 3marks
14
What is Tyndall effect? Give an example. 2marks
15
Describe the formation of a spectrum of white light by a glass prism. 3marks
16
A student is sitting in the last bench of the classroom and finds it difficult to read the writing on the blackboard. What is the defect of vision he is suffering from? Suggest the corrective measure. 3marks
17
Why does the sky appear dark to an astronaut at very high altitudes? 2marks
18
Explain the reason for the advanced sunrise and delayed sunset by atmospheric refraction. 3marks
19
(a) What is meant by the far point of the eye? (b) What is the far point of a person suffering from myopia? 2marks
20
Name the phenomenon responsible for the blue colour of the sky and for the reddish appearance of the sun at sunrise and sunset. 2marks
Answers
1
Myopia is the defect of vision in which a person cannot see distant objects clearly. The image of distant objects is formed in front of the retina. This is corrected by using a concave lens. The concave lens diverges the incoming parallel rays from the distant object. These diverged rays then enter the eye and are focused on the retina by the eye lens, thus correcting the defect.Ray Diagram: A concave lens is placed in front of the eye. Parallel rays from a distant object are diverged by the concave lens and then focused on the retina by the eye lens.Role of the lens: The concave lens provides the necessary divergence so that the rays after passing through the eye lens converge on the retina.
2
Dispersion of white light is the splitting of white light into its constituent colours when it passes through a medium like a glass prism. When white light enters a prism, each colour present in it bends through a different angle. This is because the refractive index of the prism material is different for different wavelengths (colours) of light. Violet light (shorter wavelength) bends the most, and red light (longer wavelength) bends the least. As a result, the different colours emerge from the prism along different paths, forming a spectrum of colours (VIBGYOR).
3
(a) The scattering of light in the atmosphere is caused by the molecules of air and other fine suspended particles like dust and smoke.(b) The molecules of air and other fine particles in the atmosphere are more effective in scattering light of shorter wavelengths (blue and violet) than light of longer wavelengths (red). When sunlight passes through the atmosphere, blue light is scattered more strongly in all directions than red light. This scattered blue light reaches our eyes from all parts of the sky, making the sky appear blue.
4
Danger signals are red because red light is scattered the least by fog, smoke, or dust particles in the atmosphere. Since red light is scattered the least, it can be seen clearly from a distance even in adverse weather conditions, making it suitable for danger signals.
5
A rainbow is a natural spectrum of sunlight formed by the dispersion of sunlight by tiny water droplets suspended in the atmosphere after rain.Formation:
1
Sunlight enters a water droplet. Refraction and dispersion occur at the surface as light enters from air to water. White light splits into its constituent colours.
2
Internal reflection occurs at the back surface of the water droplet.
3
Refraction occurs again as light emerges from the water droplet back into the air.Ray Diagram: The diagram shows sunlight entering a raindrop, undergoing dispersion and refraction, then internal reflection at the back, and finally emerging as a spectrum of colours due to further refraction. The observer's eye receives these dispersed colours.Processes involved: Refraction, dispersion, and internal reflection.
6
A rainbow is a natural phenomenon caused by the dispersion of sunlight by raindrops. When sunlight enters a raindrop, it first refracts and disperses into its constituent colours. This light then undergoes total internal reflection at the back of the raindrop and finally refracts again as it emerges from the raindrop. As a result, the sunlight is split into its spectrum of colours, forming a rainbow.Ray Diagram: Shows sunlight entering a raindrop, bending and splitting into colours, reflecting internally, and bending again as it exits, producing a spectrum.Phenomena involved: Refraction, dispersion, and internal reflection.
7
(a) The least distance of distinct vision is the minimum distance at which an object can be seen clearly and distinctly by the eye without strain. For a young adult with normal vision, it is approximately 25 cm.(b) A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. This is because the eye lens converges the light rays to form an image in front of the retina. A concave lens is used to correct myopia. The concave lens diverges the incoming light rays from the distant object before they enter the eye. The eye lens then focuses these diverged rays on the retina, thereby correcting the defect.
8
(a) The power of accommodation of the eye is its ability to adjust the focal length of the eye lens so as to focus images of objects at varying distances distinctly on the retina.(b) When viewing distant objects, the ciliary muscles relax, making the eye lens thinner and increasing its focal length. When viewing nearby objects, the ciliary muscles contract, making the eye lens thicker and decreasing its focal length. This adjustment of focal length allows the eye to focus on objects at different distances.
9
(a) Myopia (near-sightedness) is the defect where a person can see nearby objects clearly but not distant ones. Hypermetropia (far-sightedness) is the defect where a person can see distant objects clearly but not nearby ones.(b) Myopia is corrected using a concave lens of suitable power, which diverges the light rays so they focus on the retina. Hypermetropia is corrected using a convex lens of suitable power, which converges the light rays more strongly to focus on the retina.
10
At sunrise and sunset, the Sun is near the horizon. The sunlight has to travel a longer distance through the Earth's atmosphere to reach our eyes. During this long journey, most of the blue light (shorter wavelengths) is scattered away by the atmospheric particles. The remaining red light (longer wavelengths) reaches our eyes, making the Sun appear reddish.
11
Stars twinkle because they are very far away and appear as point sources of light. As starlight enters the Earth's atmosphere, it undergoes continuous refraction due to variations in the density and temperature of atmospheric layers. This causes the apparent position of the star to fluctuate slightly, leading to variations in the amount of light reaching our eyes, hence they twinkle. Planets, being much closer to Earth, appear as extended sources. Light from different parts of a planet reaches our eyes from slightly different directions. While light from one part might be refracted slightly away from our eye, light from another part might be refracted slightly towards our eye. These effects average out, so the brightness of the light from planets remains relatively constant, and they do not twinkle.
12
(a) The person is suffering from hypermetropia (far-sightedness).(b) This defect arises because the focal length of the eye lens is too long, or the eyeball has become too small. As a result, the light rays from a nearby object are focused behind the retina instead of on it.(c) A convex lens of appropriate power is required to correct hypermetropia. The convex lens adds to the converging power of the eye lens, helping to focus the image on the retina.
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[Diagram of the human eye with labels: Cornea, Iris, Pupil, Ciliary Muscles, Eye Lens, Retina, Optic Nerve]
14
Tyndall effect is the scattering of light by particles in a colloid or a suspension, making the path of light visible. For example, when a beam of sunlight passes through a smoke-filled room or a dense forest canopy, the path of the beam becomes visible due to the scattering of light by the particles of smoke or water droplets.
15
When a beam of white light passes through a glass prism, it splits into its constituent colours: violet, indigo, blue, green, yellow, orange, and red (VIBGYOR). This phenomenon is called dispersion. This happens because each colour of light has a different wavelength, and the prism bends each colour through a different angle. Violet light, having the shortest wavelength, bends the most, while red light, having the longest wavelength, bends the least. These dispersed colours emerge from the prism at slightly different angles, forming a coloured band or spectrum on a screen.
16
The student is suffering from myopia (near-sightedness). This is because he cannot see objects at a distance (like the blackboard) clearly.Corrective Measure: A concave lens of suitable power should be used to correct this defect.
17
The sky appears dark to an astronaut at very high altitudes because there are very few atmospheric particles to scatter sunlight. Scattering of light is responsible for the blue colour of the sky. At high altitudes, where the atmosphere is thin, the scattering effect is negligible.
18
Due to atmospheric refraction, the Sun appears to rise about 2 minutes earlier than its actual crossing of the horizon and to set about 2 minutes later. When the Sun is just below the horizon, the light rays from it travel from rarer to denser medium and bend towards the normal. Hence, the Sun appears to be at an elevated position, causing an apparent sunrise before the actual sunrise. Similarly, at sunset, the Sun appears above the horizon for about 2 minutes after it has actually set.
19
(a) The far point of the eye is the farthest point up to which the eye can see objects clearly without strain. For a normal eye, the far point is at infinity.(b) For a person suffering from myopia, the far point is nearer than infinity. They cannot see distant objects clearly because the image is formed in front of the retina.
20
The phenomenon responsible for the blue colour of the sky is the scattering of light by atmospheric particles (Rayleigh scattering). The phenomenon responsible for the reddish appearance of the sun at sunrise and sunset is also scattering of light, where longer wavelengths (red) are less scattered and reach our eyes.
