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Why do we use blue colour solution after washing the clothes?

After washing clothes we use blue color for brightening the clothes As the white clothes are deficient in ultra-violet wavelengths, this can be rectified by using violet or blue colors solutions.

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28 Draw ray diagrams to show image formation.
Object at C – concave mirror
Object beyond 2f – convex lens
Object at infinity – concave lens. Write the nature of the image in each case. 29 Light, of wavelength of 500 nm in air, enters a glass plate of refractive index 1.5. 
Find   
(i) speed    
(ii) frequency and     
(iii) wavelength of light in glass. 
Assume that the frequency of light remains the same in both media.? 31 How is critical angle related to the refractive index of the medium? 32 The refractive index of dense flint glass is 1.65, and for alcohol it is 1.36, with respect to air. What is the refractive index of dense flint glass with respect to alcohol?