3. (a) Explain briefly how the eye is able to distinguish between light of diffe

Question

3. (a) Explain briefly how the eye is able to distinguish between light of different wavelengths (b) () Explain the difference between photopic vision and scotopic vision. (i) At dusk the petals of a red flower appear to be much duller than the surrounding leaves. Explain, in terms of the rod and cone cells in the retina, the reasons for this observation. (d) Explain why the red flower is most clearly seen when looked at sideways rather than directly. . A student measures the aperture of the iris of one of her eyes as 2.0 mm in sunlight and 7.0 mm in moonlight. The intensity at her eye of sunlight is 106 times greater than the intensity of moonlight.

Explanation / Answer

(a) We are able to see only a minimal part of light spectrum which is called as visible spectrum. As light enters the eye, it strikes the inner surface of eye called as retina. On retina, we have light sensing cells called as cones and rodes. Out of these, cones are the one which are responsible for distinguishing between lights of different wavelengths. The strike of light on the cones initiate a rection which sends an impulse signal to the brain. Cones are basically red, green and blue. depending on the wavelength of light, they become active.

(b) (i) In simple words, a photopic vision is for well lit conditions. We can say that our photopic vision becomes active in morning time and scotopic comes into action when night falls. in terms of luminance, scotopic vision comes into action between 10-3 to 101/2 cd/m2. A photopic vision is associated with cones and scotopic vision is associated with rodes. Scotopic can distinguish shapes but not colours while photopic can do both.

(ii) As red has the largest wavelength, the eye is not able to distiniguish between red and surrounding green colour of leaves efficiently. As we already know that at dusk time, scotopic vision comes into action. It means rodes cells become more active than cones and rodes are more sensitive to intensity rather than wavelength. It results in dullness of red petals than surrounding green leaves. We can follow this order....VIBGYOR - violet, indigo. blue, green, yellow, orange, red.

(d) red flowers are most clearly seen when looked at sideways in low intensity conditions only and in low intensity light, rodes are the mian players and most of the rodes cells are located away from the principal axis. This is the reason to look sideways.

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