Why is a normal eye not able to see clearly the objects placed closer than 25cm?

A person needs a lens of power, -5.5 dioptres for correcting his distant vision. For correcting his near vision, he needs a lens of power, +1.5 dioptres. What is the focal length of the lens required for correcting

1. Distant vision, and
2. Near vision?

How does the convex eye-lens differ from the ordinary convex lens made of glass?

Name the atmospheric phenomenon due to which the sun can be seen above the horizon about two minutes before actual sunrise.

Why do we see a rainbow in the sky only after rainfall?

How many colours are there in a full spectrum of white light? Write the various colours of spectrum in the order, starting with red.

In the figure given below a narrow beam of white light is shown to pass through a triangular glass prism. After passing through the prism it produces a spectrum XY on a screen.

1. State the colour seen at X and Y.
2. Why do different colours of white light bend through different angles with respect to the incident beam of light?

What happens to the eye when you enter a darkened cinema hall from bright sunshine? Give reason for your answer.

Your friend can read a book perfectly well but cannot read the writing on blackboard unless she sits on the front row in class.

1. Is she short-sighted or long-sighted?
2. What type of lenses-converging or diverging-would an optician prescribe for her?

Why does the sun appear red at sunrise?