Refraction Chapter 4
What is Refraction of Light? The bending of light when it goes from one medium to another is called Refraction.
The bending of light when it goes from one medium to another is called Refraction. No refraction occurs when the light strikes the glass at right angles
Terms used to describe Refraction
Light from a ray box Refracting on entering and leaving a glass block
As the angle of incidence increases, so does the angle of refraction, but they do not increase proportionally.
When light travels from a rarer to a denser medium it is refracted towards the normal. When light travels from a denser to a rarer medium it is refracted away from the normal.
The Laws of Refraction of Light 1st Law: The incident ray, the normal at the point of incidence and the refracted ray all lie in the same plane. 2nd Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. i.e. Where n is a constant
1st Law of Refraction: The incident ray, the normal at the point of incidence and the refracted ray all lie in the same plane.
The second law of refraction
Snell’s Law Snell’s Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. i.e. where n is a constant. i is the angle of incidence and r is the angle of refraction (Note: Snell’s law is the same as the 2nd law of refraction.)
What is meant by the Refractive Index of a material? The Refractive Index of a material is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light travels from air into that material. Substance n Air 1.0003 1 Water 1.33 Glass ( Crown ) 1.5 Glass ( Flint ) 1.6 Diamond 2.4 (Strictly speaking from a vacuum, but the difference is negligible).
Due to refraction an object in a liquid appears at a lesser depth than it actually is.
The apparent depth decreases as you view the object more from the side The apparent depth decreases as you view the object more from the side. The apparent depth is greatest when the object is viewed from vertically above it.
Real Depth and Apparent Depth An object is viewed from air through another medium and the line of view is perpendicular to the surface of the medium. What is the relationship between the refractive index of the medium, n, the real depth and the apparent depth? n = Real Depth Apparent Depth
Refractive Index in terms of Relative Speeds
Speed of Light in Water = Speed of Light in Air Refractive Index of Water
Explain the term: Critical Angle When light travels from a denser to a rarer medium the angle of incidence whose angle of refraction is 90o is called the Critical Angle (C).
What is Total Internal Reflection? When light going from a denser to a rarer medium strikes the rarer medium at an angle greater than the critical angle, it does not enter the second medium. It is all reflected back in the denser medium. This is Total Internal Reflection.
To Demonstrate Total Internal Reflection Set up the equipment as shown and slowly increase the angle of incidence i.
When the critical angle is reached the refracted ray skims along the flat face of the glass. Increase i further and total internal reflection occurs. Critical Angle Total Internal Reflection
Refractive Index of a medium
Turning a ray of light through 90o with a prism
Turning a ray of light through 180o with a prism
Optical Fibres A very thin transparent rod (usually of glass) through which light can travel by total internal reflection is called an Optical Fibre.
A very fine optical fibre passing through the eye of a needle
If two Optical Fibres touch, light may escape from one and pass into the other.
Optical fibres are usually coated with glass of lower refractive index. This ensures that light always stays in the inner denser glass even if two fibres touch each other. It also prevents damage to the surface of the denser inner fibre.
Endoscopic Image of a Human Foetus
Hot air has a lower refractive index than cool air Hot air has a lower refractive index than cool air. A ray of light from the sky heading towards the road travels from a denser to a rarer medium and will be refracted away from the normal. It bends as shown. If, at some point, the angle of incidence is greater than the critical angle the ray will undergo total internal reflection and be reflected upwards. An observer will see an image of the sky in the ground. This is the shiny puddle, i.e., the mirage. An nearby object may be seen upside down in the puddle due to the same effect.
To Verify Snell’s Law and measure Refractive Index
To Verify Snell’s Law and measure Refractive Index
The Graph is a Straight Line through the Origin. This verifies that: sin i sin r i.e. it verifies Snell’s Law