Reflection and Refraction of Light: Understanding Mirrors and Lenses

Posted on Aug 26, 2024 in Physics

Reflection of Light

Reflection: The bouncing back of rays of light from a polished and shiny surface is called reflection or reflection of light. It is similar to the bouncing back of a football after colliding with a wall or any hard surface.

Types of Images

Real Image: An image formed in front of the mirror that can be obtained on a screen.

Virtual Image: An image formed behind the mirror that cannot be obtained on a screen.

A plane mirror always forms a virtual and erect image. The distance of the image and that of the object is equal from the mirror. The image formed by a plane mirror is laterally inverted.

Key Terms in Spherical Mirrors

Pole (P): The center of the reflecting surface of a spherical mirror.

Center of Curvature (C): The center of the sphere of which the reflecting surface of a spherical mirror is a part.

Radius of Curvature (R): The radius of the sphere of which the reflecting surface of a spherical mirror is a part.

Aperture: The diameter of the reflecting surface of a spherical mirror.

Principal Axis: An imaginary line passing through the center of curvature and pole of a spherical mirror.

Principal Focus (F): The point on the principal axis at which parallel rays coming from infinity converge after reflection.

Focal Length (f): The distance from the pole to the focus. Focal length is equal to half of the radius of curvature.

Rules of Reflection for Spherical Mirrors

• A ray parallel to the principal axis appears to diverge from the principal focus after reflecting from the surface of a convex mirror.
• A ray passing through the center of curvature returns on the same path after reflecting from the surface of a concave mirror.
• A ray oblique to the principal axis goes obliquely after reflecting from the pole of both concave and convex mirrors, and at the same angle.

Uses of Spherical Mirrors

Concave Mirrors

• As reflectors in electric torches, headlights of vehicles, searchlights, etc.
• As shaving mirrors to produce a larger image of the face.
• Used by dentists to see a larger image of teeth.
• As reflectors in solar furnaces.

Convex Mirrors

• Used in rearview mirrors of vehicles.
• Used on hairpin bends on roads.

Refraction of Light

A ray of light changes its direction when it enters from one medium to another. This happens because the speed of light is different in different media.

Refractive Index: The extent of the change of direction of light in a given pair of media. It is a relative value of the speed of light in the given pair of media.

Absolute Refractive Index: When one medium is taken as a vacuum, the refractive index of the second medium with respect to the vacuum is called the Absolute Refractive Index.

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Rules of Refraction

• A ray parallel to the principal axis appears to diverge from the principal focus after reflecting from the surface of a convex mirror.
• A ray passing through the center of curvature returns on the same path after reflecting from the surface of a concave mirror.
• A ray oblique to the principal axis goes obliquely after reflecting from the pole of both concave and convex mirrors, and at the same angle.
• A ray passing through the center of curvature returns on the same path after reflecting from the surface of a concave mirror.

Sign Conventions

1. All distances will be measured from the pole of the mirror.
2. In the direction of the incident ray, all distances will be taken as positive (+ve), and in the opposite direction to the incident ray, all distances will be taken as negative (-ve).
3. Above the principal axis will be positive, and below it will be negative.

Refraction of Light: The change of direction of light because of the change of medium. The ray of light changes its direction because of the difference in speed in different media.

When a ray of light enters from a rarer medium into a denser medium, it bends towards the normal at the point of incidence. Conversely, when a ray of light enters into a rarer medium from a denser medium, it bends away from the normal.

Magnification Formula:

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Image Formation by Concave Mirrors

Image Formation by Convex Mirrors