Class 10 Science Chapter 9: Light Reflection & Refraction – Total Internal Reflection
Total Internal Reflection (TIR) is an important phenomenon related to the refraction of light. It occurs when light travelling through an optically denser medium strikes the boundary with an optically rarer medium at an angle greater than the critical angle. In this article, we will understand Total Internal Reflection Class 10, its conditions, critical angle, examples, applications, and important questions for CBSE Class 10 Science Chapter 9 – Light Reflection & Refraction. What is Total Internal Reflection? When a ray of light travels from an optically denser medium to an optically rarer medium, it normally bends away from the normal. However, when the angle of incidence is increased beyond a certain limiting angle, called the critical angle, the refracted ray disappears and the entire light is reflected back into the denser medium. This phenomenon is called Total Internal Reflection. Definition Total Internal Reflection is the phenomenon in which a ray of light travelling from an optically denser medium to an optically rarer medium is completely reflected back into the denser medium when the angle of incidence is greater than the critical angle. Conditions Necessary for Total Internal Reflection There are two essential conditions for Total Internal Reflection: 1. Light must travel from a denser medium to a rarer medium The light must move from a medium having a higher refractive index to a medium having a lower refractive index. For example: TIR does not occur when light travels from a rarer medium to a denser medium. 2. Angle of incidence must be greater than the critical angle The angle of incidence must be greater than the critical angle (C). Therefore: Angle of incidence > Critical angle i > C Only then does complete internal reflection take place. What is the Critical Angle? The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium becomes 90°. In other words, when light travels from a denser medium to a rarer medium and the refracted ray just grazes the surface, the corresponding angle of incidence is called the critical angle. It is represented by C. At the critical angle: Angle of refraction = 90° For Total Internal Reflection: Angle of incidence > Critical angle What Happens When the Angle of Incidence Changes? Suppose light travels from glass to air. There are three important cases: Case 1: Angle of incidence is less than the critical angle When: i < C The ray is refracted into the rarer medium and bends away from the normal. Case 2: Angle of incidence is equal to the critical angle When: i = C The refracted ray travels along the boundary between the two media. Therefore: r = 90° Case 3: Angle of incidence is greater than the critical angle When: i > C No refracted ray emerges into the rarer medium. The complete ray is reflected back into the denser medium. This is Total Internal Reflection. Basis of Comparison Ordinary Reflection Total Internal Reflection (TIR) Meaning Light is reflected back from a surface when it strikes the surface. Light is completely reflected back into the denser medium at the boundary of two transparent media. Direction of Light It can occur for light travelling in any direction towards a reflecting surface. Light must travel from an optically denser medium to an optically rarer medium. Angle of Incidence No specific condition regarding the critical angle is required. The angle of incidence must be greater than the critical angle. Critical Angle Critical angle is not required. Critical angle is an essential concept for TIR. Refraction In ordinary reflection from a transparent surface, some light may be transmitted into the second medium. No refracted ray emerges into the rarer medium when TIR occurs. Amount of Light Reflected Generally, only a part of the incident light may be reflected. Ideally, 100% of the incident light is reflected back into the denser medium. Where It Occurs At a reflecting surface or boundary. At the boundary between an optically denser and an optically rarer transparent medium. Main Examples Reflection from a plane mirror, polished metal surface, etc. Optical fibres, sparkling of diamonds and totally reflecting prisms. Important Condition Follows the law of reflection: angle of incidence = angle of reflection. Requires denser → rarer medium and angle of incidence > critical angle. Refractive Index and Critical Angle For light travelling from a medium of refractive index n₁ to a rarer medium of refractive index n₂, where n₁ > n₂, the critical angle is related by: sin C = n₂ / n₁ For the special case of light travelling from glass to air, the refractive index of air is approximately 1. Therefore: sin C = 1 / n where n is the refractive index of glass with respect to air. Why Does Total Internal Reflection Occur? When light travels from an optically denser medium to an optically rarer medium, the refracted ray bends away from the normal. As the angle of incidence increases, the angle of refraction also increases. At a particular angle of incidence, the refracted ray makes an angle of 90° with the normal. This angle is called the critical angle. If the angle of incidence is increased further, the light can no longer emerge into the rarer medium. Instead, it is completely reflected back into the denser medium. This is Total Internal Reflection. Examples of Total Internal Reflection in Daily Life Total Internal Reflection has several important applications. 1. Optical Fibre One of the most important applications of Total Internal Reflection is in optical fibres. An optical fibre consists mainly of a core surrounded by cladding. The core has a higher refractive index than the cladding. Light entering the fibre undergoes repeated Total Internal Reflection and travels through the fibre over long distances. Uses of optical fibres Optical fibres are used in: 2. Sparkling of Diamonds Diamonds sparkle brightly because of Total Internal Reflection. Diamond has a very high refractive index and therefore a small critical angle. Light entering a diamond
