Define reflection
When light travelling in a medium encounters a boundry leading to a second medium, part of the
incident light is returned to the first medium from which it came.
Information of reflection and an example
When light travels through a medium (like air, glass, or water) and hits a boundary between that medium and another (like when light moves from air to water), part of the light is reflected back into the original medium. This process is called reflection.
Reflection happens because the light waves encounter a change in the properties of the material at the boundary. These properties could be things like the refractive index (how much the medium slows down light) or density. When light hits the boundary, some of it may be “bounced back” into the first medium, while the rest might pass into the second medium, possibly bending in the process (which is called refraction).
Here’s what happens in more detail:
Incident light: Light approaching the boundary is called the incident light.
Reflection: Some of this incident light gets reflected back into the first medium. The angle at which the light hits the boundary is called the angle of incidence. The angle at which the light is reflected is called the angle of reflection, and according to the law of reflection, these two angles are always equal. In other words, the angle of incidence equals the angle of reflection.
Refraction: The rest of the light may pass into the second medium and bend at the boundary, depending on the difference in the refractive indices of the two media. This bending is called refraction, and it follows Snell’s law.
The amount of reflection depends on several factors:
The angle of incidence: The closer the light hits the boundary at a perpendicular (90°) angle, the less it will be reflected and more likely it will pass into the second medium.
The difference in refractive indices: If the two media have a large difference in refractive indices, more light will be reflected.
So, reflection is basically the bouncing back of light from a surface or boundary when it encounters a change in the medium’s properties. This principle is used in many everyday phenomena, like mirrors or even the “shininess” of certain materials!
🟦 Info Card: Mirrors — Overview
Any smooth surface can act as a mirror.
Mirrors can be plane (flat) or curved.
Past: Made by coating glass with silver.
Present: Made by depositing a thin film of aluminium on a polished surface.
Mirrors may be back-coated or front-coated depending on their purpose.
Function: Redirect light rays and form images of objects.
What materials were used in the past and present to make mirrors?
Past: Silver coating on glass.
Present: Thin film of aluminium on a polished surface.
What are the main types of mirrors and their basic function?
Types: Plane and curved.
Function: Redirect light rays and form images of objects.
🟦 Info Card: Reflection at Plane Surfaces
There are two types of reflection:
Regular (specular) reflection
Occurs when a parallel beam of rays strikes a smooth, polished surface (like a mirror).
The rays are reflected in one direction.
Diffuse reflection
Occurs when a parallel beam of rays strikes an irregular or rough surface (like paper or a road).
The rays are scattered in many directions.
Importance: Diffuse reflection allows us to see non-luminous objects.
Example: At night, roads are visible because car headlights are diffusely reflected from the rough road surface.
What is the difference between regular (specular) and diffuse reflection?
Regular reflection: From a smooth, polished surface; rays are reflected in one direction.
Diffuse reflection: From a rough or irregular surface; rays scatter in all directions.
Why is diffuse reflection important in everyday life?
It enables us to see objects that do not emit light themselves.
Example: Roads are visible at night because car headlights are diffusely reflected from the rough surface.
🌟 1. Incident Ray (AB)
This is the incoming ray of light — the one that travels towards the mirror.
In your example, ray AB is the incident ray.
It strikes the mirror at a specific point, called the point of incidence.
- Point of Incidence (B)
This is the exact point on the mirror’s surface where the incident ray meets the mirror.
In your figure, point B is where the light ray hits the surface.
At this point, reflection begins.
🌟 3. Reflected Ray (BC)
After hitting the mirror, the light ray bounces off — this outgoing ray is called the reflected ray.
In your example, ray BC represents the reflected ray, which leaves the mirror at an angle.
🌟 4. Normal
If we draw an imaginary line that is perpendicular (at 90°) to the mirror surface at the point of incidence (B), that line is called the normal.
It helps us measure angles and understand how light reflects.
🌟 5. Angle of Incidence (i)
This is the angle between the incident ray (AB) and the normal at the point of incidence.
It tells us how “steeply” the light ray strikes the mirror.
🌟 6. Angle of Reflection (i′)
This is the angle between the reflected ray (BC) and the normal.
It shows how steeply the light bounces off the mirror.
🌟 Law of Reflection
The relationship between these angles is very simple but fundamental:
Angle of incidence (i) = Angle of reflection (i′)
This law means light reflects symmetrically — the path in is mirrored by the path out.
First law
The incident ray, the reflected ray and the
normal lie in one plane which is perpendicular
to the surface
Second law
The angle of reflection is equal to the angle of
incidence, i = i’
What does FOAIBAPM mean
Formation of an Image by a Plane Mirror
🧩 1. The setup - FOAIBAPM
A point source of light, called S, is placed in front of a plane mirror.
From this point, light rays spread out (diverge) in different directions — let’s say three rays: SA, SB, and SC.
These rays hit the mirror at points A, B, and C.
🪞 2. Reflection of the rays - FOAIBAPM
According to the law of reflection, the angle of incidence (i) equals the angle of reflection (i′) for each ray.
The ray SA, which hits the mirror perpendicularly, will be reflected straight back along its own path (because the angle of incidence is 0°).
The other rays (SB and SC) will reflect off the mirror at equal but opposite angles, along new paths (BD and CE).
📐 3. Extending the reflected rays backward - FOAIBAPM
When you extend the reflected rays (BD and CE) backward behind the mirror, they appear to meet at a point called S′.
Your eyes perceive these rays as if they are coming from S′, even though they are not actually passing through that point.
This point S′ is where the image of the object (S) appears to be located.
🔺 4. Geometrical proof (Triangles SAB and S′AB) - FOAIBAPM
By geometry:
Both triangles SAB and S′AB share the same size and shape (they are congruent).
They share a common side (AB).
The angles of incidence and reflection are equal (i = i′).
Both have a right angle at the mirror’s surface (∠SAB = ∠S′AB = 90°).
Because the triangles are congruent, SA = S′A.
That means:
The distance of the object from the mirror (SA) = The distance of the image behind the mirror (S′A)
👁️ 5. What the eye sees - FOAIBAPM
When you look into the mirror:
The reflected rays (BD, CE, etc.) enter your eyes.
Your brain traces them backward in straight lines.
They appear to come from behind the mirror at point S′.
But no light actually comes from S′ — it’s only a virtual image.
🧠 6. Summary: Key properties of an image formed by a plane mirror - FOAIBAPM
The image is virtual (cannot be formed on a screen).
The image is upright.
The image is laterally inverted (left and right are swapped).
The image distance = object distance from the mirror (SA = S′A).
The size of the image = size of the object.
