State one role of ciliary muscles in the human eye?


Ciliary muscles holds the eye in its place. Also, it changes the shape of the lens by contracting or relaxing the muscles. This process is known as accommodation.

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What is the colour of the sky during day time? Give reason for it?


The colour of the clear sky is blue during the day time. This is because of the scattering of light by the atmosphere. The spectrum of white light comprises of different colours. Out of these colours, the blue colour gets scattered the most because of its shorter wavelength as compared to the long wavelength of red colour.

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What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the Sun appears reddish at sunrise?


Scattering of light is the phenomenon in which a part of the incident light is dispersed in different directions.

Appearance of blue sky: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So, they are more effective in scattering light of shorter wavelengths at the blue end than
light of longer wavelengths at the red end. Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes, and hence, the clear sky appears blue.

Sun appears reddish at sunrise: At the time of sunrise and sunset, when the Sun is near the horizon, sunlight travels a greater distance through the atmosphere to reach us. During this, most of the shorter wavelengths present in it are scattered away from our line of sight by the molecules of air and other fine particles in the atmosphere. So, light reaching us directly from the rising or setting Sun consists mainly of the longer wavelength red colour because of which the Sun appears red. Thus, at sunrise and sunset, the Sun as well as the surrounding sky appears red.
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A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.

Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE.
(i) Write the name and cause of the phenomenon observed.
(ii) Where else in nature is this phenomenon observed.
(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light.




i) The name of the phenomenon is dispersion of light. White light is comprised of seven colours. Dispersion of light occurs because these colors travel at different speeds through glass prism. Hence, different colors undergo different deviations on passing through prism.

ii) This phenomenon is observed in rainbow formation. Rainbow is caused due to dispersion of sunlight by water droplets present in the atmosphere. Rainbow is always formed in a direction opposite to the sun.

iii) The constituents of white light is a sequence of colours given by Violet, Indigo, Blue, Grren, Yellow, Orange and Red (VIBGYOR). Red light bends the least and violet light bends the most.
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Draw a labelled ray diagram to illustrate the dispersion of a narrow beam of white light when it passes through a glass prism.




When a ray of light passes through a glass prism, different colour components of white light travel through different velocities and come out from prism along different paths.
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