## Ray Optics and Optical Instruments  Important Questions for CBSE Class 12 Physics Optical Instrument

1.Near Point (D) The minimum distance at which an object can be seen most distinctly without any strain. For a healthy normal eye, it is 25 cm.

2.Simple Microscope (Magnifying glass) It is a converging lens of small focal length.

3.Compound Microscope It consists of two convex lenses, one objective of very small focal length with short aperture and one eyepiece, E of moderate focal length and large aperture

4.Reflecting Astronomical Telescope These telescopes form image free from chromatic aberration and spherical aberration.

5.Refracting Astronomical Telescope It consists of an objective lens of a large focal length (f0) and large aperture, also an eye lens of small aperture and focal length.

### 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.

(i) Reflecting telescopes have high           resolving  power due to a large aperture of mirrors.

(ii) 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.

Limitations of refracting telescope over a reflecting  type telescope.

(i) Refracting telescope suffers from chromatic aberration uses large sized lenses.

(ii) 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]

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:

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

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

(iii) A mirror reflects the light, so the material that is made from does not  have to be transparent ultraviolet light reflects equally well.

6.Two convex lenses of same focal length but of apertures Ax and A2 (A2 < Ar), are used as the objective lenses in two astronomical telescopes having identical eyepieces. What is the ratio of their resolving power? Which telescope will you prefer and why? Give reason. [Delhi 2011]

7.Define the resolving power of a  telescope. Write any two advantages of a reflecting telescope over a refracting telescope.                     [Delhi 2010 c]

8.Draw a labelled ray diagram of  an astronomical telescope in the near point position. Write the expression for its magnifying power.           [All India 2008]

Ans.  Ray diagram of an astronomical telescope in the near point position is

9.Draw a ray diagram of an  astronomical telescope in the normal adjustment position. State two drawbacks of this type Of telescope. [Delhi           2008]

Drawbacks

(i) Image formed by these telescopes have chromatic and spherical aberration.

(ii) The length of telescope is very large in normal adjustment.

10.Draw a ray diagram of compound microscope. Write the expression for its magnifying power.    [Delhi           2008]
Ans.

11.(i) Draw a labelled ray diagram  showing the formation of a final image by a compound microscope at least distance of distinct vision, (ii) The total magnification produced by a compound microscope is 20. The magnification produced by the eyepiece is 5. The microscope is focussed on a certain object. The distance between the objective and eyepiece is observed to be 14 cm. If least distance of distinct vision is 20 cm. Calculate the focal length of the objective and the eyepiece.  [Delhi 2014 C]

12.Draw a labelled ray diagram of a refracting telescope. Define its magnifying power and write the expression for it.  Write two important limitations of a refracting telescope over a reflecting type telescope.                     [All India 2013]

Limitations of refracting telescope over a reflecting  type telescope.

(i) Refracting telescope suffers from chromatic aberration uses large sized lenses.

(ii) It is difficult and expensive to make such large sized lenses.
13.Draw a ray diagram showing the image formation by a compound microscope. Hence, obtain expression for total magnification when the image is formed at the infinity. [Delhi 2013]

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.

14.A compound microscope uses an  objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens. Calculate the magnifying power of the compound microscope. Also, calculate the length of the microscope.     [All India 2011]

Ans.

NOTE

1. The separation between objective and eye lens is known as length of microscope.
2. The image formed by objective is an object for eye lens.

15.A giant reflecting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece lens of focal length 0 cm is used, find the angular magnification of the telescope.  If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.42 x 106 m and the radius of the lunar orbit is 3.8 X 108  m.                       [All India 2011]

Ans.

16.Two convex lenses of focal length  20 cm and 1 cm constitute a telescope. The telescope is focused on a point which is 1 m away from the objective. Calculate the magnification produced and the length of the tube if the final image is formed at a distance 25 cm from the eyepiece.                 [Delhi 201 ic]

Ans.

17.The objective of an astronomical  telescope has a diameter of 150 mm and a focal length of 4.00 m. The eyepiece has a focal length of 25.00 mm. Calculate the magnifying and resolving power of telescope. (X = 6000 A for yellow Colour).     [Delhi 2011C]

18.(i) Draw a       neat       labelled       ray  diagram of an astronomical telescope in normal adjustment. Explain briefly its working.

(ii) An astronomical telescope uses two lenses of powers 10 D and ID. What is its magnifying power in normal adjustment?  [All India 2010]

Ans.

Refer to ans. 2.

The image formed by objective lens must fall on the focus of eye lens in order to form final image at infinity.

19.i) Draw a       neat       labelled       ray  diagram of a compound microscope. Explain briefly its working.

(ii) Why must both the objective and the eyepiece of a compound microscope have short focal lengths?    [All India 2010]

Ans.  (i)  Refer to ans. 3.

The magnification by compound microscope is two step process.

Firstly, the objective gives a magnified image of the object and after that the eye piece produces the angular magnification.

20.Draw a schematic diagram of a  reflecting telescope (Cassegrain). Write two important advantages that the reflecting telescope has over a refracting type.        [Foreign 2010]

Ans.

21.Explain with the help of a ray diagram, the working of an astronomical telescope. The magnifying power of a telescope in its normal adjustment is  If the length of the telescope is 105 cm in this adjustment, find the focal lengths of the two lenses.  [Ail India 2010C]

Ans.  For figure refer to ans. 2.

The parallel beam of light from distant object get focused in focal plane of objective at angle a. This image A’B’ acts as an object for eye lens which lie between optical centre of eye lens and its focus. Eyepiece form virtual, erect, magnified image A” B” at least distant of distinct vision.

### 5 Marks Questions

22.Draw a labelled ray diagram  showing the image formation of a distant object by refracting telescope,

Deduce the expression for its magnifying power when the final image is formed at infinity.

(ii) The sum of focal lengths of the two lenses of a refracting telescope is 105 cm. The focal length of one lens is 20 times that of the other. Determine the total magnification of the telescope when the final image is formed at infinity. [All India 2014]

Ans.

23.Define magnifying power of a telescope. Write its expression.  A small telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eyepiece.       [Delhi 2012]

Ans.The magnifying power of a telescope is equal to the ratio of the visual angle subtended at the eye by final image formed at least distance of distinct vision to the visual angle subtended at naked eye by the object at infinity.

24.How is the working of a telescope different from that of a microscope? The focal lengths of the objective and eyepiece of a microscope are 1.25 cm and 5 cm. respectively. Find the  position of the object relative to the objective in order to obtain an angular magnification of 30 in normal adjustment.                                             [Delhi 2012]

25. Draw a ray diagram to show the working of a compound microscope.  Deduce an expression for the total magnification when the final image is formed at the near point.

In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eyepiece has a focal length of 5 cm and the final image is formed at the near point. Estimate the magnifying power of the microscope. [Delhi 2010]

Ans.For figure Refer to ans. 3.

The objective lens forms real, inverted magnified image A’B’ of object AB in such a way that AB’ fall some where between pole and focus of eye lens. So, A’B’ acts as an object for eye lens and its virtual magnified image A” B” formed by the lens.

The magnifying power of a compound microscope is defined as the ratio of the visual angle subtended by final image at eye ((3) and the visual angle subtended by object at naked eye when both are at the least distance of distinct vision from the eye.

26.(i) (a) Draw a labelled ray  diagram to show the formation of image in an astronomical telescope for a distant object.

(b) Write the three distinct advantages of a reflecting type telescope over a refracting type telescope.

(ii) A convex lens of focal length 10 cm is placed coaxially 5 cm away from a concave lens of focal length 10 cm. If an object is placed 30 cm in front of the convex lens. Find the position of the final image formed by the combined system. [All India 2009]

27.Draw the labelled ray diagram for the formation of image by an astronomical telescope.

Derive the expression for its magnifying power in normal adjustment. Write two basic features which can distinguish between a telescope and a compound microscope. [Foreign 2009]

28.(i) Draw a ray diagram for the formation of image by a compound microscope. Define

its magnifying power. Deduce the expression for the magnifying power of the microscope.

(ii) Explain

• Why must both the object and the eyepiece of a compound microscope have short focal lengths?
• While viewing through a compound microscope, why should our eyes be  positioned not on the eyepiece but a short distance away from it for best viewing?           [Foreign 2008]

Ans.  (i) For figure Refer to ans. 3.

For magnifying power refer to ans. 25

(ii) (a) Refer to ans. 19 (ii).

(b) When eyes are positioned at short distance away from eyepiece, then the image formed at infinity can be seen which is more suitable and comfortable for viewing by the relaxed eye