Revision: Optics CUET (UG) Optics

The phenomenon due to which a parallel beam of light traveling through a certain medium, on striking some polished surface, bounces off from it, as a parallel beam, in some other direction, is called regular reflection.

The principal axis is the straight line passing through the pole and the centre of curvature.

The bouncing of light by any smooth or polished surface is called.

The distance of the image from the pole of the mirror is called the image distance (v).

The distance of the principal focus from the pole is called the focal length (f).

In a spherical mirror, the distance of the object from its pole is called the object distance (u).

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

The change in the direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. The refraction of light is essentially a surface phenomenon.

When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.

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Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.

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The bending of the light ray from its path in passing from one medium to the other medium is called 'refraction' of light.

Refracted light is the part of light enters into the other medium and travels in a straight path but in a direction different from its initial direction and is called the refracted light.

When a ray of light propagates from a denser medium to a rarer medium, the angle of incidence for which the angle of refraction is 90° is called the critical angle.

When rays of light parallel to the principal axis of a mirror are incident on it, the rays after reflection either converge at a point or appear to diverge from a point. The distance of that point from the pole of the mirror is known as the focal length of the mirror.

Power of a lens is defined as the ability of a lens to bend the rays of light. It is given by the reciprocal of focal length in metre.

The power of a lens is a measure of the deviation produced by it in the path of rays refracted through it.

The deviation of the incident light rays produced by a lens on refraction through it, is a measure of its power.

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The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P.

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The power (P) of a thin lens is equal to the reciprocal of its focal length (f) measured in metres.

The angular separation between the two extreme colours (violet and red) in the spectrum (which is obtained by passing a beam of white light through a prism) is known as angular dispersion.

Angular magnification or magnifying power of an optical instrument is defined as the ratio of the visual angle made by the image formed by that optical instrument (β) to the visual angle subtended by the object when kept at the least distance of distinct vision (α).

The resolving power of an astronomical telescope is defined as the reciprocal of the smallest angular separation between two point objects whose images can just be resolved by the telescope.

R.P = `(1.22 lambda)/D`

Resolving power is the ability of the telescope to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve.

Wavefront is defined as the locus of all the points in space that reach a particular distance by a propagating wave at the same instant.

A wave front is defined as a surface of constant phase.

\[\frac {1}{v}\] + \[\frac {1}{u}\] = \[\frac {1}{f}\]

Magnification (m) = \[\frac{\text{Height of the image (}h'\text{)}}{\text{Height of the object (}h\text{)}}\] = \[\frac {h'}{h}\]

Magnification in terms of object and image distances:

Magnification (m) = \[\frac {h'}{h}\] = -\[\frac {v}{u}\]

Power of lens (in D) = \[\frac{1}{\text{focal length (in metre)}}\]

or

P = \[\frac {1}{f}\]

or

P = \[\frac {1}{f (m)}\]

Power of a Lens in a Medium:

P = (n₂ - n₁)\[\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)\] = \[\frac {n_1}{f}\]

• Reflection occurs when light bounces off a smooth surface like a mirror, following fixed laws.
• Plane mirrors always form virtual, erect, and same-sized images that are laterally inverted.
• Curved surfaces (like a spoon) act as spherical mirrors, changing the image size and orientation depending on the object's position.

• Red colour of the Sun at sunrise and sunset is due to maximum scattering of blue light and least scattering of red light in the atmosphere.
• The blue colour of the sky is due to the greater scattering of blue (or violet) light by air molecules because of its short wavelength.
• The black colour of the sky in the absence of atmosphere occurs because there is no scattering of sunlight.
• Red light is used for danger signals because it has the longest wavelength and is scattered the least, so it can be seen from a far distance.

• Myopia is a vision defect in which distant objects appear blurry, while near objects are seen clearly.
• This occurs because the image of distant objects forms on the retina.
• The far point is not at infinity but is shifted closer to the eye.
• Causes include increased curvature of the cornea/lens or elongation of the eyeball.
• Corrected using a concave lens of negative power, which diverges light rays to focus the image on the retina.