Ray Optics And Optical Instruments

- Learn important concepts of the chapter

Laws of refraction

According to laws of refraction (Snell's Laws):

1. The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for the pair of given media. This constant is called the refractive index of the second medium w.r.t. the first medium.


1. When light ray is incident normally, only speed changes and direction of  light remains the same.
2. When light ray passes from rarer medium to denser medium, it bends towards the normal.
3. When light ray passes from denser medium to rarer medium, it bends away from the normal.

Observe the refraction through a rectangular glass slab

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Consider a rectangular glass slab ABCD as shown in fig. For refraction at AB
for refraction at CD, 
But, , from eq. (1) and(2), we get
Angle of incidnece = Angle of emergence

Apparent depth and real depth

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Real Depth is actual distance of an object beneath the surface, as would be measured by submerging a perfect ruler along with it.

Apparent depth in a medium is the depth of an object in a denser medium as seen from the rarer medium. Its value is smaller than the real depth.

Total internal reflection (TIR)

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When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection.


Total internal reflection is defined as the complete reflection of a light ray at the boundary of two media, when the ray is in the medium with greater refractive index.


Refraction of rays at spherical surfaces

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Combination of thin lenses in contact

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Consider two lenses A and B of focal length and placed in contact with each other.
An object is placed at O on  the common principal axis. The lens A produces an image at and this image acts as the object for the second lens B. The final image is produced at as shown in figure.
PO = u, object distance for the first lens (A),
PI = v, final image distance and
, image distance for the first lens (A) and also object distance for second lens (B).
For the image produced by the first lens A,
 .... (1)
For the final image I, produced by the second lens B,
 ... (2)
Adding equations (1) and (2),
  ... (3)
If the combination is replaced by a single lens of focal length F such that it forms the image of O at the same position I, then
... (4)
From equations (3) and (4),
...... (5)
This F is the focal length of the equivalent lens for the combination.

Simple microscope

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A simple microscope or a magnifying glass is an optical device used to obtain magnification of small objects for better clarity of vision. It is a convex lens of short focal length mounted in a lens holder. To obtain magnification, it is placed at a small distance from the object and virtual image is formed. 
It is used to see very small letters and figures, by watchmakers, etc.

Compound microscope

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Compound microscope is an optical device used to obtain very large values of magnification. It is used to see microscopic objects like microorganisms. It comprises of two convex lenses and magnification occurs in both of them. Its basic components are:

1. Objective: Convex lens placed near the object. Object is placed just beyond the focal point. Image formed in real, inverted and magnified.
2. Fine adjustment screw: This is used to adjust the position of objective lens. A small movement in this causes a lot of change in magnification.
3. Eyepiece: Convex lens placed near the eye. It forms a virtual object of the real image formed by the objective lens. Hence, its position is designed such that image from the objective falls between its focus and centre. 
4. Rough adjustment screw: This is used to form  a clear image and focus at the correct part of the object to be seen.
Working Principle: 
Light from a light source (mirror or electric lamp) passes through a thin transparent object. The objective lens produces a magnified real image first image) of the object. This image is again magnified by the ocular lens (eyepiece) to obtain a magnified virtual image (final image), which can be seen by eye through the eyepiece. As light passes directly from the source to the eye through the two lenses, the field of vision is brightly illuminated.


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Telescope is an optical device used to form the image of very far objects near the eye so that it appears bigger. It is hence used to obtain angular magnification. It is used to study the astronomical objects.  It consists of two convex lenses. Image formed by objective lens is real and inverted and is formed at its focal point. It is large in size to capture more light and improve magnification. Image formed by objective lens serves as the object for eyepiece lens.  Image formed by eyepiece lens is virtual and erect. 

Resolving Power of Optical Instruments

Resolving Power of Optical Instruments: A quantity that characterizes the ability of optical instruments to produce separate images of two points of an object that are close to each other. The smallest linear or angular distance between the two points at which their images begin to merge is called the linear or angular limit of resolution. The inverse quantity usually serves as a quantitative measure of the resolving power.

Because of the diffraction of light at the edges of optical components, even in an ideal optical system (that is, one without aberrations), the image of a point is not a point but a central disk of light surrounded by rings, which are alternately dark and light in monochromatic light and rainbow-colored in white light.