Ray Optics And Optical Instruments

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Total internal reflection in optical fibres
Optical fibres use the principle of total internal reflection. The light ray incident at an angle greater than critical angle undergoes T.I.R. and stays trapped inside the optical fibre. This technique of transmitting information using light energy is very efficient as the 100% of the energy get reflected at the interface of dense-to-rare medium.

Optical Fibers

3 mins

Construction and Working of Various Telescopes: The telescopes are used to provide angular magnification of distant objects. The light from the distant objects is gathered using the objective which has a large diameter.The objective can either be:

a convex lens
a concave mirror.

Due to this difference, telescopes can be classified as:

refracting telescopes
reflecting telescopes

Reflecting telescopes are preferred in cases where the lenses tend to be very heavy and difficult to make and support by their edges. A mirror weighs much less than a lens of equivalent optical quality, and can be supported over its entire back surface, not just over its rim. Thus, a concave mirror in reflecting telescopes gives the same optical output with an easier construction and mounting. Also, chromatic aberration is not a problem in reflecting telescopes.

Construction and Working of Refractive Telescope

3 mins

Construction and Working of Reflecting Telescope

2 mins

Chromatic Aberration and Spherical Aberration
Despite of the well formulated equations, the practical lenses show a deviation from their ideal behaviour. The equations we use are valid only for thin lenses and for small apertures. So the lenses in real show two types of distortions, or aberration in image formation. They are of two types: (a) Chromatic aberration and (b) Spherical aberration

(a) Chromatic aberration is caused by the dependence of a lens's refractive index on colour (wavelength). The lens is more powerful for violet (V) than for red (R), producing images with different locations and magnifications. That's why we consider thin lenses for ideal image formation.

(b) Spherical aberration is present when the outer parts of a lens do not bring light rays into the same focus as the central part. Images formed by the lens at large apertures are therefore unsharp but get sharper at smaller apertures. That's why we consider paraxial rays and assume small aperture for ideal image formation.

Chromatic Aberration

3 mins

Spherical Aberration

3 mins