L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION PHYSICS - GRADE: VIII REFRACTION OF LIGHT

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1 L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION PHYSICS - GRADE: VIII REFRACTION OF LIGHT REFRACTION When light travels from one transparent medium to another transparent medium, it bends from its original path. This phenomenon of bending of light is called Refraction. Refraction takes place at the surface of separation of the two media. BENDING OF LIGHT TRAVELLING FROM ONE OPTICAL MEDIUM TO ANOTHER The bending of light ray as it passes obliquely from one optical medium to another takes place because light travels with different speeds in different media. DIFFERENCE BETWEEN REFLECTION OF LIGHT AND REFRACTION OF LIGHT Reflection of Light 1. Reflection of light is defined as the bouncing of light after striking a surface. 2. In Reflection of Light, the angle of incidence is equal to the angle of reflection. 3. In Reflection of Light, the light ray is reflected back into the same medium. Refraction of Light Refraction of light is defined as the phenomenon of bending of light when it travels from one transparent medium to another transparent medium. In Refraction of Light, the angle of incidence is not equal to the angle of refraction. In Refraction of Light, the light ray travels from one transparent medium to another transparent medium. TERMS RELATED TO REFRACTION OF LIGHT

2 1. INCIDENT RAY The ray which falls on the surface of separation to enter into the other medium is known as the Incident Ray. 2. REFRACTED RAY The ray obtained in the second medium after refraction is known as the Refracted Ray. 3. NORMAL A Normal is an imaginary straight line perpendicular to the refracting surface. 4. ANGLE OF INCIDENCE The angle between the Incident Ray and the Normal at the point of incidence is known as the Angle of Incidence. 5. ANGLE OF REFRACTION The angle between the Refracted Ray and the Normal at the point of incidence is known as Angle or Refraction. RULES OF REFRACTION 1. When a ray of light passes obliquely from a rarer medium to a denser medium, it always bends towards the normal. In such case, the angle of refraction is smaller than the angle of incidence. 2. When a ray of light passes obliquely from a denser medium to a rarer medium, it always bends away from the normal. In such case, the angle of refraction is greater than the angle of incidence. 3. When a ray of light passes from one medium to another medium at right angle to the surface separating the two media, it does not bend. Instead, it goes in its original direction only. LAWS OF REFRACTION 1. First Law of Refraction The incident ray, the normal at the point of incidence and the refracted ray, all lie in the same plane. 2. Second Law of Refraction For a given pair of media and given colour of light, the ratio of the since of angle of incidence to the sine of angle of refraction is a constant. sin i = constant sin r This constant is denoted by the symbol µ. The Second law is also known as Snell s Law. The constant (µ) is known as the Refractive Index of the second medium with respect to the first medium.

3 REFRACTION THROUGH A PARALLEL SIDED GLASS SLAB Observation 1. The incident ray, the normal and the refracted ray, all are drawn on the same sheet of paper or, lie on the same plane. Hence, the first law of refraction is verified. 2. The ratio of sin i / sin r is found to be constant for two given media, that is, glass and air. Hence, Snell s law is verified (when the experiment is repeated with different angles of incidence). 3. The angle of incidence and the angle of emergence are always equal. In other words, the emergent ray is always parallel to the incident ray. 4. The emergent ray shifts from the path of the incident ray. Thus, when a ray of light passes through a rectangular glass slab, it emerges parallel to the incident ray and is shifted from its original path. REFRACTION OF LIGHT THRHOUH A PRISM Observation: 1. Angle of incidence + Angle of emergence = Angle of prism + Angle of deviation. 2. The incident ray and emergent ray are not parallel to each other. The emergent ray makes an angle with the extended incident ray (angle of deviation).

4 3. The angle of deviation depends upon the angle of incidence. 4. When angle of incidence is increased gradually, the angle of deviation first decreases and attains a minimum value and then increases. The minimum value it acquires is called as the angle of minimum deviation (d m ). Conditions during minimum deviation: The angle of incidence and the angle of emergence are equal to each other. The ray of light, travelling through the prism, is parallel to the base of the prism. The angle of minimum deviation depends on: DISPERSION OF LIGHT Angle of prism Material of prism Angle of incidence Wavelength and colour of light used. The splitting of white light into its constituent colours as it passes through a refracting medium is known as Dispersion. CAUSE OF DISPERSION The speed of light in a medium is different for the light of different colours. Hence, the refractive index of the medium is different for the light of different colours. Due to which, they deviate from their path differently. SPECTRUM Spectrum is the band of seven colours obtained on a white screen when white light passes through a prism and slits into its constituent colours. VIBGYOR constitutes the Spectrum. RAINBOW In nature, rainbow is an example of dispersion of white light. After the rain, a large number of small droplets of water remain suspended in the air. Each drop acts like a small prism. When sunlight passes through these drops, it splits into seven colours. With a large number of water droplets contributing to this phenomenon, we get to see a huge continuous band of seven colours displayed in the sky.

5 LIGHT DISPERSES THROUGH A PRISM BUT NOT THROUGH A GLASS SLAB The different coloured light with different frequency bend differently as long as the prism surfaces on which light is incident and emerging are not parallel. But when the surfaces are parallel, the effect of dispersion from the incident surface is undone by the other surface. A glass slab acts in this fashion as it could a join of two prisms which dispersed light from one surface is again undone by the other surface. Hence, the light disperses through a prism but not through a glass slab. CONCEPT MAP