8th Class Science Light NCERT Summary - Reflection of Light

Reflection of Light

• A ray of light, the incident ray, travels in a medium.
• When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium.
• This means it is directed backward into the first medium.

Laws of Reflection of Light

• The angle of incidence and angle of reflection are equal.
• The incident ray, reflected ray and normal to the point of reflection lie in the same plane.

Reflection of Light in a Plane Mirror

• The image formed by a plane mirror is always virtual, upright, and of the same shape and size as the object it is reflecting.
• When an object is placed in front of a plane-mirror, an image is formed behind the mirror at a distance equal to the distance of the object from the mirror.
• The focal length of plane mirror is infinity because there is no divergence or convergence and run parallel forever. The focal length of plane mirror does not exist.
• Emergency vehicles are usually reverse-lettered so the lettering appears normal in the rear view mirror of a car.
• The minimum mirror height necessary to see one's full image is of half-length mirror.
• The point on the mirror surface, where the incident ray strikes is called point of incidence.
• The ray which is directed towards the reflecting surface is called the incident ray.
• The ray which is sent back into the medium is called the reflected ray.
• The perpendicular drawn at the point of incidence to the surface of mirror is called normal.
  • Angle between the incident ray and normal is called angle of incident.
  • Angle between the reflected ray and normal is called angle of reflection.

Spherical Mirror

• A spherical mirror is actually a mirror whose surface forms a part of a hollow sphere. The sphere can be made from any type of polished metal surface like glass.
• Convex Mirror: A mirror with a spherical surface and reflecting from the exterior of the curvature is called a convex mirror. A convex mirror is also known as Diverging Mirror as it diverges the incident rays after reflection.
• Concave Mirror: A mirror with a spherical surface and reflecting from the interior of the curvature is called a concave mirror. A concave mirror is also known as Converging Mirror as it converges the incident rays after reflection.

Reflection at Spherical Surface

• The geometric centre of a spherical mirror is called its pole.
• The centre of the hollow sphere for which the mirror is a part, is called the centre of curvature.
• The line joining the centre of curvature and the pole is the principle axis.
• A light ray incident of a spherical mirror, after reflection appears to pass through the principal focus in the case of a convex mirror and passes through the focus in the case of concave mirror.
  • The diameter of the spherical mirror gives the measure of its aperture.
  • Rays that lie close to the principal axis are called paraxial rays.
• Rays that are far from the principal axis do not converge to a single point. The fact that a spherical mirror does not bring all parallel rays to a single point is known as spherical aberration.
• Spherical aberration can be minimised by using a mirror whose height is small compared to the radius of curvature.
• When paraxial light rays that are parallel to the principal axis strike a convex mirror, the rays appear to originate from a focal point behind the mirror.
• Virtual image formed by plane mirror is laterally inverted means left and right side is reversed and same as the object size and in front of the mirror as it is kept from the mirror, it is directly proportional.
• Whereas image formed by convex mirror is virtual, diminished and cover larger area and used as a rear view mirror.

Laws of Refraction

• When a ray of light travelling through a transparent medium encounters a boundary leading into another transparent medium, part of the ray is reflected and part of the ray enters the second medium.
  • The ray that enters the second medium is bent at the boundary.
  • This bending of the rays is called refraction.
• The incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in one plane.
• For any two given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
• The above law is called Snell's law after the scientist Willebrord Snell (also called Snellius) who first formulated it.
• Thus \[\frac{sin\,i}{\sin \,r}=\,\text{constant}\,=\,\mu \]
• This constant value\[\mu \]is the refractive index of the second medium with respect to the first medium.

Dispersion of Light

• If a white light is passed through the prism the light splits up into a series of colours. This shows that white light is actually made up of many other colours - a fact first shown by Newton in 1666. The distribution of colours produced when white light is dispersed by a prism is spectrum.
• White light is dispersed because different colours are affected differently by the passage into glass. Specifically, the different colours travel at different speeds through the glass (Because of this, each colour has its own index of refraction).
• Red light is refracted the least when it enters the block, and violet light is refracted the most. This effect is called dispersion. However, this difference is corrected when the light leaves the block, so the effect is not noticeable.
• Refraction of white light through a glass prism, which has sides that are not parallel, causes this difference in refraction of different colours of light to be exaggerated - so we see a spectrum of light emerging from the prism.
• The speed is related to the wavelength as \[(V=\lambda )\] where V = velocity of light, V = frequency and \[V\lambda \]= velocity of light wave. This indicates that the colour with the highest speed (red) has the longest wavelength, while the colour with the slowest speed (violet) has the shortest wavelength.
• Because frequency is inversely proportional to wavelength, we can also determine the relative frequencies. Red would have the lowest, while violet has the highest.
  • The primary colours are red, blue and green.
• Additive Colour mixing involves multiple sources of light with different colours in each source. Three additive primary colours are red, green and blue (RGB).
• Subtractive Colour mixing involves a single source of light with different colours absorbing various wavelengths of the colour spectrum. The subtractive primary colours are cyan blue, magenta red, and yellow (CMYK).