Class11 Physics Notes( OPTICS)

                       OPTICS

Q.Why is there no dispersion of monochromatic light ?

Ans:

When white líght is passed through a lens (or a prism), it is splitted into sevendifferent colours which make up white light. This is because different colours(wavelengths) have different speed in the medium (lens or prism) and angle of reaction is different for difierent colours. But, when monochromatic light (light of particular wavelength) is passed through a lens (or a prism), it doesn't get splitted and hence, no dispersion.

Q.At sunrise or sunset the sun looks almost reddish.Why?

Ans:

At the time of sunrise or sunset, the sun is near horizon and the light from thesun has to traverse larger thickness of atmosphere that what it covers, when the sun is overhead. Due to this, more of the blue light is scattered away and the sun appears reddish.

Q.The clouds are generally white. Why?

Ans:

The water drops and dust are present in the atmosphere. Such larger particlesdo not scatter light according to Rayleigh law of scattering. They scatter light of all colours almost equally and hence the clouds are generally white.

Q.Why is sky blue ? Explain.

Ans: 

The blue colour of the sky is due to the scattering of light. When light from the sun travels through the earth's atmosphere, the different wavelength of light get scattered from their path through different amounts obeyingRayleigh's law of scattering(i.e Amount of scattering is proportional to fourth power ofWavelength.) Since the wavelength of the blue colour is approximately half the wavelength of the red colour, the scattering of blue colour light is  about 16 times more than that of red light. Due to this, blue colour predominates and the sky appears blue.

Q. Explain why a mirror cannot give rise to chromatic aberration.

Ans: Chromatic aberration is caused due to the dispersion of light. But there is no dispersion of light when it is reflected by a miror because all component colours of white light incident on a mirror are reflected by the same angle. Hence, miror cannot give rise to chromatic aberration.

Q. Why should the aperture of the objective of a oicroscope be smal

Ans: 

A microscope is used to magnify a small obiect. If the aperture of the objective of the microscope is large, it will collect more light from the surrounding objects of the objecty. lf the aperture of the objective is small, it collects light only from the object to be viewed and hence it is seen cléarly. Therefore, the aperture of the objective of a microscope should be small.

Q.Very distant stars are not visible to the eye but when viewed through a telescope, they become visible, why ?

Ans:

Since the aperture of eye lens is very small, it collects only small amount of light from the distant stars so that the light thus collected is not enough to excite the retina. Hence, distant stars are not visible to unaided eye. The aperture of the objective of the telescope is made large so that it collects more light from the stars and forms a bright image on the retina. As a result, the distant stars become visible to the eye when viewed through a telescope.

Q.Why is the aperture of the objective of a telescope is made large as compared to that of its eyepiece ?

Ans:

If the aperture of the objective is made large, it collects more light from the object to be viewed and the light thus collected may be enough to excite the retina to form the image of the object. Also the resolving power and field view of a telescope increases with aperture. But, the aperture of eyepiece  made small so that all the rays coming out of it may enter the pupil of the eye.