Optical Instrument

2 Marks Questions

1.Draw a ray diagram of a reflecting type telescope. State two advantages of this telescope over a refracting telescope. [ Delhi 2014 C; All India 2008C; Delhi 2008]

Ans.

Objective

mirror (C)

distant star rays from Parallel mirror Secondan Eyepiece

Advantages

1. Reflecting telescopes have high resolving power due to a large aperture of mirrors.
2. Due to availability of paraboloidal mirror, the image is free from chromatic and spherical aberration.

2.Draw a schematic diagram of refracting telescope. Write its two important limitations. [Delhi 2014c]

Ans.

Objective

mirror (C)

distant star rays from Parallel mirror Secondar Eyepiece

Limitations of refracting telescope over a reflecting type telescope.

1. Refracting telescope suffers from chromatic aberration uses large sized lenses.
2. It is difficult and expensive to make such large sized lenses.

3.Draw a ray diagram for the formation of image by a compound microscope. Write the expression for total magnification when the image is formed at infinity. [Delhi 2014c]

Ans. A compound microscope consists of two convex lenses coaxially separated by some distance. The lens nearer to the object is called the objective. The lens through which the final image is viewed is called the eyepiece. The focal length of objective lens is smaller than eyepiece.

4.A convex lens of focal length 25 cm is placed coaxially in contact with a concave lens of focal length 20 cm. Determine the power of the combination. Will the system be converging or diverging in nature? [Delhi 2013] Given, focal length of convex lens, <math>f_1 = +25 \text{ cm} = +0.25 \text{ m}</math> and focal length of concave lens, <math>f_2 = -20 \text{ cm} = -0.20 \text{ m}</math> Equivalent focal length of convex and concave lens, <math>F = \frac{1}{f_1} + \frac{1}{f_2} = \frac{1}{25} + \frac{1}{-20} = -\frac{1}{100}</math> <math display="block">\therefore F = -100 \text{ cm} = -1 \text{ m}</math>

Now, the power of lens <math>P = \frac{1}{f}</math> For convex lens, <math>P_1 = \frac{1}{f_1} = \frac{1}{0.25}</math> For concave lens, <math>P_2 = \frac{1}{f_2} = \frac{1}{-0.20}</math> Hence, power of the combination <math>P = P_1 + P_2 = \frac{1}{0.25} + \frac{1}{-0.20} = \frac{100}{25} + \frac{100}{-20}</math> <math display="block">=\frac{400-500}{100}=\frac{-100}{100}=-1D</math> Here, the focal length of the combination <math>= 100 \text{ cm} = -1 \text{ m}</math> Since, the focal length is in negative, so the system will be diverging in nature. (2)

5.Draw a schematic arrangement of a reflecting telescope (Cassegrain) showing how rays coming from a distant object are received at the eyepiece. Write its two important advantages over a refracting telescope. [Delhi 2013C]

Ans. Diagram of a reflecting telescope (Cassegrain) is shown as below:

Advantages of reflecting telescope over a refracting telescope are given as below:

1. A reflecting telescope reflects all wavelengths of light at the same angle, so there are no colour halos.

建设了成立。1800年的统一人的处理。可是 Objective Secondary mirror mirror \

Eyepiece

1. A mirror has only one surface to be figured, so it is easier to control the shape.