$A$ person is advised to wear spectacles with concave lenses. What type of defect of vision is he suffering from?

What is dispersion of light? Name the $(i)$ component of white light that deviates the least,$(ii)$ component of white light that deviates the most,while passing through a glass prism.

Sushil went to an eye specialist for a check-up. The doctor prescribed him to use spectacle lenses of $+0.5 \text{ D}$ power.
$(a)$ Name the defect of vision he is suffering from.
$(b)$ Find the focal length of the spectacle lens.

Which color is produced by mixing blue and red?

Mention the factor on which scattering of light depends. Why does the sky appear dark in space?

How many times is the wavelength of red light compared to the wavelength of blue light (in $times$)?