Ray Optics

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Optics

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Optic is a branch of physics which deals with the study of light

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Ray optics

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Ray optics deals with the propagation of light in straight lines. in order to explain some of the phenomenons exhibited by light like reflection refraction dispersion and scattering

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Wave optics

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In wave optics light propagate in the form of waves. The phenomenons like diffraction interference and polarization can be explained only with the help of wave optics based on wave theory

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Reflection

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Bouncing back of light from the polish to surface is known as reflection of light. The polished surface is known as mirror

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Refraction of light

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The phenomenon of change in the part of light as it passes obliquely from one transparent medium to another is called refraction of light

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Cause of refraction of light

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Different medium have different optical density is, due to this speed of light differs in different medium. The speed of light is more in rarer medium. Since light has to cover equal distance in equal intervals of time, due to the decrease in a speed of light in the density medium. Light takes the shortest route. Thus it bends.

Refraction is only for light waves which are falling at an interface oblique valley if the light is falling at an interface normally there is no change the path of light that is it does not undergo refraction.

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Snell's law

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1. The incident ray, refracted Ray and the normal to the point of interface line in the same plane

2. The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant called the refractive index of the second medium with respect to the first medium

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Total internal reflection (TIR?

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When light travels from denser to rare medium it Bends away from the normal. When the angle of incidence in the density medium is increased the corresponding angle of refraction in the rarer medium is also increased. on increasing the angle of incidence, the refracted ray passes along the interface that is angle of refraction becomes 90 degree

The angle of incidence in the density medium for which the angle of refraction becomes 90 degree is critical angle. if angle of incidence is again increased the ray will be totally an internally reflected into the denser medium known as total reflection

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Conditions for TIR

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1. Light should travel from denser to rare medium

2. The angle of incidence in the denser medium should be greater than critical angle

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Mirage

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On hot summer days, the air near the ground becomes hot then the hair at high levels. Hot air is less dense and has smaller refractive index than cooler air. As a result light from a tall object such as a tree passes through a medium whose refractive index decreases towards the ground. So the light bends away from the normal and undergoes TIR. To a distance observer the light appears to be coming from somewhere below the ground this causes inverted images and the phenomenon is called Miraj

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Diamond

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The brilliancy of diamond is due to TIR. The critical angle for diamond air interface is 24.4 degree is very small. therefore once light enters a diamond it undergo TIR. by cutting diamond suitable multiple TIR can be made to occur

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Totally reflecting Prisms

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Presence designed bend light by 90 or 180 degree make use of TIR. These prisms are used to invert images without changing their size. (Critical angle is equal to 42 degree)

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Optical Fibre

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Optical fibre make use of the phenomenon TIR. These are fabricated with high quality quartz fibres. Each fibre consists of a core and a cladding.

When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated TIR along the length of the fibre and finally comes out the other end. Since light undergoes TIR at each stage there is no appreciable loss on the intensity of light signal

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Power of the lens

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The power of a lens is a measure of the degree of convergence or divergence of the light rays falling on it. A convex lens of short of focal length bends the light rays through a larger angle. SI unit of power is diopter.

for convex lens power is positive and for concave lens power is negative

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Simple Microscope

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A skmple microscope is a converging lens (convex lens) of small focal length. The object is placed at a distance less than the focal length of the lens. A magnified virtual images forming at D.

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Simple Microscope (Normal Adjustment)

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When we see the image at D, it causes some strain in the eye. When the image is formed at infinity, it is more suitable and comfortable for viewing.

Here the magnification is angular magnification instead of linear magnification. It is defined as the ratio of the angle formed by the image at infinity to the anger formed by the object when situated at D with the eye.

The magnification is one less than the magnification when the images formed at the near point.

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Compound Microscope

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A simple microscope has a limited magnification. For much larger magnifications we use two lenses one is near to the object called the objective and the other near to the eye called the eye piece. Objective forms a real inverted and magnified image of the object which acts as a virtual object for eyepiece and the final image is forming as D.

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Astronomical telescope

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The telescope is used to provide angular magnification of distant objects. It has an objective and an eyepiece. The objective has a large of focal length and much larger aperture than eyepiece to gather the light from the distant object.

The magnifying power is the ratio of angles subtended by the image with the eye to the anger subtended by the object with the eye.

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Reflecting telescope

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It consist of a large concave mirror as the primary mirror and small convex mirror as a secondary mirror. The parallel rays from the distant object are reflected by the large concave mirror. They are again reflected by a convex mirror and are conversed to a point form in the final image.

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Advantages of reflecting telescope

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1. High gathering power

2. It is free from the defects such as spherical or chromatic aberration.

3. A mirror weighs less

4. Less expensive