SPHERICAL MIRRORS
These are mirrors with spherical reflecting surfaces. There are two types:
- Concave mirror: Its reflecting surface is curved inwards (faces towards the center of the sphere).
- Convex mirror: Its reflecting surface is curved outwards.
- Pole (P): It is the center of the reflecting surface of a spherical mirror. The reflecting surface is a part of a sphere.
- Centre of curvature (C): It is the center of the sphere of which the spherical mirror is a part. It is not part of the mirror and lies outside the reflecting surface. The centre of curvature lies in front of a concave mirror and behind a convex mirror.
- Radius of curvature (R): It is the radius of the sphere of which the mirror forms a part. The distance PC = radius of curvature.
- Principal axis: It is the imaginary straight line passing through the pole and centre of curvature of a spherical mirror. It is normal to the mirror at its pole.
Principal Focus (F) and Focal Length (f)
- Direct the reflecting surface of a concave mirror towards the Sun and direct the reflected light onto a paper.
- Move the paper to find a bright, sharp spot of light on it. Wait for a few minutes. The paper burns and catches fire.
- This is because the light from the Sun is converged (concentrated) at a point on the paper as a bright spot (image of the Sun). This point is the focus of the concave mirror. It produces heat and ignites the paper.
- The distance of this image from the position of the mirror is the focal length of the mirror. This can be represented as a ray diagram.
- When several rays parallel to the principal axis fall on a concave mirror, the reflected rays meet/intersect at a point on the principal axis of the mirror. This point is called the principal focus (F) of the concave mirror.
- In a convex mirror, the reflected rays appear to come from a point on the principal axis. This point is called the principal focus of the convex mirror.
- The distance between the pole and the principal focus of a spherical mirror is called the focal length (f).
- The reflecting surface of a spherical mirror is generally spherical and has a circular outline. The diameter of the reflecting surface is called its aperture (the distance MN). We consider only spherical mirrors whose aperture is much smaller than its radius of curvature.
- For spherical mirrors of small apertures, the radius of curvature is equal to twice the focal length (R = 2f), i.e., the principal focus lies midway between the pole and the centre of curvature.
Select a Topic 👇
1. Reflection of Light 2. Spherical Mirrors 3. Image Formation by Spherical Mirrors 4. Sign Convention, Mirror Formula & Magnification 5. Refraction of Light 6. Refraction through a Rectangular Glass Slab 7. Refractive Index, Refraction by Spherical Lenses 8. Image Formation by Lenses 9. Lens Formula & Magnification, Power of Lenses