# First Year Physics Optical Instruments (iv) For total internal reflection the angle of incidence must be the critical angle.(a) greater than (b) smaller than (c) equal to (d) half of

Change the way you learn with Maqsad's classes. Local examples, engaging animations, and instant video solutions keep you on your toes and make learning fun like never before!

##### (iv) For total internal reflection the angle of incidence must be the critical angle.(a) greater than (b) smaller than (c) equal to (d) half of

Example 4.2 Light passes from air into diamond with an angle of incidence of 45^{\circ} . Calculate the angle of refraction if the index of refraction of diamond is 2.42 .

An astronomical telescope having magnifying power of 5 consist of two thin lenses 24 \mathrm{~cm} apart. Find the focal lengths of the lenses.

(v) A convex lens is(a) thinner at the centre (b) thicker at the centre (c) a diverging lens (d) plane throughout

4.2 The speed of light in water is 2.25 \times 10^{5} \mathrm{~km} / \mathrm{s} . What is the index of refraction of water?

Example 4.6 Calculate the distance of the object from a convex lens having a focal length of 15 \mathrm{~cm} if the magnification is 3 and the image is real.

A converging lens of focal length 5.0 \mathrm{~cm} is used as a magnifying glass. If the near point of the observer is 25 \mathrm{~cm} and the lens is held close to the eye. Calculate (i) the distance of the object from the lens.

A compound microscope has lenses of focal length 1.0 \mathrm{~cm} and 3.0 \mathrm{~cm} . An object is placed 1.2 \mathrm{~cm} from the object lens. If a virtual image is formed 25 \mathrm{~cm} from the eye calculate the separation of the lenses and the magnification of the instrument.

(vii) In a photographic camera a convex lens is used because it (a) produces a real and small image (b) gives virtual image (c) produces an enlarged image (d) forms an image equal in size to the object

(v) Explain the functions of human eye. Name its different parts.

A converging tons of focal length 5.0 \mathrm{~cm} is used as a magnifying glass if the near point of the observer is 25 \mathrm{~cm} and the lens is held close to the eye calculate(ii) the angular magnification. What is the angular magnification when the final image is formed at infinity?

A simple astronomical telescope in normal adjustment has an objective of focal length 100 \mathrm{cm} and an eye piece of focal length 5.0 \mathrm{~cm} . (i) Where is the final image formed (ii) Calculate the angular magnification.

(iii) If an objcct is situated between the optical centre and the principal focus of a convex lens then its image is formed on the same side as that of the object. This image will be(a) inverted real and bigger in size (b) inverted real and of the same size as that of object (c) inverted real and smaller in size (d) erect virtual and large in size.

Example 4.5 An object is placed 12 \mathrm{~cm} from a convex lens of focal length 18 \mathrm{~cm} . Find the position and magnification of the image.

(vii) Describe the construction and action of the following:(c) Astronomical telescope.

Example 4.3 What is the index of refraction of a material if the angle of incidence in air is 60^{\circ} and the angle of refraction in the material is 34^{\circ} .

Draw sketches showing the different light paths through a single-mode and a multimode fibre. Why is the single-mode fibre preferred in telecommunications?

How the light signal is transmitted through the optical fibre?

4.8 The distance between an object and a convex lens is 18 \mathrm{~cm} . The focal length of the lens is 6 \mathrm{~cm} . Determine the nature. position and magnification by using the lens formula.( -4.5 \mathrm{~cm} virtual 0.25 times)

4.5 Find the focal length of a convex lens if (a) p=5 \mathrm{~cm} q=10 \mathrm{cm} and theimage is virtual (b) p=30 \mathrm{~cm} and the image is real.

Point object is placed on the axis of and 3.6 \mathrm{~cm} from a thin convex lens of focal length 3.0 cm. A second thin convex lens of focal length 16.0 \mathrm{~cm} is placed coaxial the first and 26.0 \mathrm{~cm} from it on the side away from the object. Find the position of the final image produced by the two lenses.

(vi) The eye and the camera are similar because the image formed in both is(a) real inverted and small (b) real erect and small (c) virtual erect and small (d) real inverted and enlarged

4.6 The focal length of a convex lens is one metre ana an object is placed at a distance of 2 \mathrm{~m} before it. Determine the position nature and magnification of the image.

(ii)Explain in some detail the refraction of light through a prism.

4.4 Pick out true and false from the following sentences.(i) If a ray of light enters normally from one medium into the other then the angle of refraction is much larger than angle of incidence.(ii) One angle of the totally reflecting prism is 90^{\circ} while the other two angles are 45^{\circ} each.(iii). The principal focus of a concave lens is always virtual.(iv) A virtual image is that which can be obtained on a screẹ.(v) The reciprocal of the focal length of a lens is called its power.(vi) The unit of the power of the lens is dioptre.(vii) Centre of curvature is the centre of that sphere of which the lens is a part.(viii) The distance between the centre of curvature and the optical centre is called focal length.(ix) The image formed by a concave lens is always real.(x) The principal focus of a convex lens is always real. .(xi) Total internal reflection always takes place when the angle of incidence is much less than the critical angle.(xii) A simple microscope is a bi-concave lens.(xiii) The image in the human eye is formed at the iris.(xiv) A compound microscope contains a convex and a concave lens.

Example 4.1The speed of light in diamond is 1.24 \times 10^{5} \mathrm{~km} / \mathrm{s} and that in air is 3.0 \times 10^{8} \mathrm{~m} / \mathrm{s} . Calculate the index of refraction of diamond.

Why would it be advantageous to use blue light with a compound microscope?

Identify the correct answer:(i) The resolving power of a compound microscope depends on;(a) The refrective index of the medium in which the object is placed.(b) The diameter of the objective lens.(c) The angle subtended by the objective lens at the object.(d) The position of an observers eye with regard to the eye lens.

Sodium light of wavelength 589 \mathrm{~nm} is used to view an object under a microscope. If the aperture of the objective is 0.90 \mathrm{~cm} (ii) using visible light of any wavelength what is the maximum limit of resolution for this microscope.