Reflection and Refraction of Light

• Lesson 3 discusses reflection and refraction of light.
• Light travels in straight lines and the formation of a shadow occurs when light cannot pass through dark objects.

Light Reflection

• Light reflection is analogous to a ball rebounding off a wall.
• Light reflection occurs when light meets a reflecting surface and rebounds.
• The "reflecting surface" is where reflection takes place.
• Light reflection is the rebounding of light waves in the same medium after hitting a reflecting surface.
• Types of light reflection are classified by the nature of the reflecting surface.

Regular (Uniform) Reflection

• Regular reflection happens when light rays hit a smooth surface.
• Incident light rays are reflected in one direction.
• Examples of smooth surfaces include plane mirrors, thin aluminum sheets (foil), and stainless steel sheets.

Irregular (Non-Uniform) Reflection

• Irregular reflection happens when light rays hit a rough surface.
• Incident light rays are reflected in multiple directions.
• Examples of rough surfaces include leaves, paper, leather, and wool.
• A clean mirror causes regular reflection, while a dirty mirror causes irregular reflection.

Laws of Light Reflection

• To understand and use the laws of light reflection, it is important to know the basic concepts.
• The incident light ray is a narrow beam represented by a straight line that intersects the reflecting surface at the point of incidence.
• The reflected light ray is a narrow beam represented by a straight line reflected from the reflecting surface at the point of incidence.
• The angle of incidence is the angle between the incident light ray and the line perpendicular (normal) to the reflecting surface at the point of incidence.
• The angle of reflection is the angle between the reflected light ray and the line perpendicular to the reflecting surface at the point of incidence.
• The angle of reflection = 40° means the angle between the reflected light ray and the line perpendicular to the reflecting surface at the point of incidence equals 40°.
• The angle of incidence = 30° means the angle between the incident light ray and the line perpendicular to the reflecting surface at the point of incidence equals 30°.
• Light reflection is governed by two laws.
• The first law of reflection states that the angle of incidence equals the angle of reflection.
• The second law states that the incident light ray, the reflected light ray, and the normal to the surface of reflection at the point of incidence all lie in one plane perpendicular to the reflecting surface.

Light Refraction

• Light refraction is like a barrel changing its path when moving from asphalt to sand and back, due to a change in velocity.
• Light refraction is the change in the light's path as it travels from one transparent medium to another due to the difference in velocity.
• Light refraction is the change of light path when it travels from a transparent medium to another transparent medium of different optical density.
• The "optical density of the medium" is its ability to refract light.
• Each medium has its own optical density
• The optical density of a medium differs from one medium to another, which changes the light velocity through such medium.
• As optical density increases, the speed of light through it decreases, and vice versa.
• The velocity of light changes from one medium to another because the optical density of a medium differs from one medium to another.

Concepts Related to Light Refraction

• Angle of incidence: The angle between the incident light ray and the normal at the point of incidence on the interface.
• Angle of refraction: The angle between the refracted light ray and the normal at the point of incidence on the interface.
• Angle of emergence: The angle between the emergent light ray and the normal at the point of emergence on the interface.
• The angle of emergence in a prism being 50° means the angle between the emergent light ray and the line perpendicular to the interface at the point of emergence is 50°.

Laws of Light Refraction

• The path of a light ray falls on the interface between two transparent media that differ in their optical densities.
• When light travels from a transparent medium of lower optical density (like air or water) to one of higher optical density (like glass), it refracts near the normal.
• When light travels from a transparent medium of higher optical density (like glass) to one of lower optical density (like air or water), the light ray refracts far from the normal.
• When a light ray falls perpendicular to the interface between two different transperant media, the light passes without refraction.
• The angle of incidence is greater than the angle of refraction when light travels from a medium of lower optical density to one of higher optical density.
• The angle of incidence is smaller than the angle of refraction when light travels from a medium of higher optical density to one of lower optical density.
• The angle of incidence is equal to the angle of refraction and equals to zero when light falls perpendicular to the interface between two media.

Factors Necessary for Light Refraction

• The presence of an interface between two different transparent media with different optical densities.
• The incident light ray falls inclined to the interface (angle of incidence ≠ zero).
• The amount of refraction depends on the optical density of each medium.

Absolute Refractive Index

• The absolute refractive index is a ratio of the velocity of light through air to its velocity through another transparent medium.
• The absolute refractive index of a medium = (Velocity of light through air) / (Velocity of light through the medium).
• The absolute refractive index of any transparent medium is always greater than one because the velocity of light through air is always greater than that through any other transparent medium.
• Glass, water, and air are examples of transparent media with different optical densities: Glass > Water > Air.
• A medium with a large optical density has a large absolute refractive index, and the velocity of light passing through it will be small.
• A diamond's ability to refract light is greater than that of glass because the absolute refractive index of diamond is greater than the absolute refractive index of glass.

Natural Phenomena

• Common natural phenomena are related to reflection and refraction of light.
• A pencil partially immersed in water appears broken due to refraction.
• Submerged objects appear at an apparent position slightly above their real position because of refraction.
• A fish in a glass bowl appears higher than its real position due to refraction.
• To pick up a coin in water, one must look at it vertically because the incident light ray perpendicular passes without refraction, so the apparent position is the real position.

Mirage

• Mirage is a natural phenomenon on desert roads, where objects appear to have inverted images as if on a wet surface.
• Mirage is caused by reflection and refraction of light in air layers with different temperatures.