21.

Jatin puts a pencil into a glass container having water and is surprised to see the pencil in a different state.

i) What change is observed in the appearance of the pencil?

ii) Name the phenomenon responsible for the change.

iii) Draw a ray diagram showing how the eyes see the pencil.

i) Where should an object be placed so that a real and inverted image of the same size as the object is obtained using a convex lens? 
ii) Draw a ray diagram to show the formation of the image as specified in the part a (i).

Why does the sun appear red at the sunrise? 

Name the subjective property of light related to its wavelength. 

i) Draw a graph between displacement and the time for a body executing free vibrations.

i) State the safe limit of sound level in terms of decibel for human hearing.

A person standing between two vertical cliffs and 480 m from the nearest cliff shouts. He hears the first echo after 3 s and the second echo 2 s later.

Calculate:

i) The speed of sound.

ii) The distance of the other cliff from the person.

What happens to the resistivity of semi-conductors with the increase of temperature? 

Find the equivalent resistance between points A and B. 

The relationship between the potential difference and the current in a conductor is stated in the form of a law.

i) Name the law.

ii) What does the slope of V-I graph for a conductor represent?