1. Laws of reflection

As the car moved along the road, Aaron looked at the shiny side-view mirror and noticed how clearly it reflected the vehicles behind. Curious, he said, “Dad, how do mirrors always show things so perfectly?”
His father smiled and replied, “That’s because mirrors follow some fixed rules, the laws of reflection. These laws decide how light bounces back when it strikes a mirror.”
 
Path of light:
When light travels from one place to another, it moves in a straight line. This straight path followed by light is called the path of light.
Ray:
A ray of light is a narrow straight line showing the direction of travel of light. It is usually represented by a straight line with an arrowhead pointing in the direction of the light’s movement.
 Beam of light:
A beam of light is a group of light rays that travel together in the same direction.
Depending on how the rays spread, a beam can be
  1. Parallel beam – rays are parallel.
  2. Converging beam – rays come together.
  3. Diverging beam – rays spread apart.
Reflection:
When light rays fall on a shiny or polished surface, such as a mirror, they bounce back into the same medium. This bouncing back of light from a surface is called reflection of light. The surface that sends the light back is known as a reflecting surface.
To understand it clearly, we use some special terms.
 
Incident Ray:
The ray of light that falls on the mirror.
 
Point of Incidence:
The point where the incident ray strikes the mirror surface.
 
Reflected Ray:
The ray of light that bounces off the mirror from the point of incidence.
 
Normal:
An imaginary line drawn perpendicular (\(90°\)) to the mirror surface at the point of incidence.
 
Angle of Incidence (i):
The angle between the incident ray and the normal.
 
Angle of Reflection (r):
The angle between the reflected ray and the normal.

To study how light reflects, we compare the angles made by these rays. 
Activity:
To observe reflection of light from a plane mirror.
 
flashlight1.svg
Light rays reflected from a plane mirror
 
Step 1: Place a white sheet of paper on the table and keep the plane mirror upright along one edge.
Step 2: Hold a stencil with three slits in front of the torch to obtain three thin rays of light.
Step 3: Direct the three rays so that they fall on the mirror at different angles.
Step 4: Mark the incident rays and the corresponding reflected rays on the paper and remove the mirror.
Step 4: At each point of incidence, draw a normal line.
Step 5: Measure the angle of incidence (i) and the angle of reflection (r) for each ray using a protractor.
 
1a.png
Ray diagram
 
Observation:
For all three rays, the angle of reflection is found to be equal to the angle of incidence.
 
Conclusion:
This confirms that the angle of incidence is always equal to the angle of reflection. 
Let us perform another activity to understand the reflection better. 
Activity:
To observe the light's reflection from a plane mirror.
 
Step 1: Place a chart paper flat on the table and keep the plane mirror upright on one edge of the chart paper.
Step 2: Shine a thin beam of light from a torch on the mirror.
Step 3: Observe the reflected beam on the same sheet, this is the reflected ray.
Step 4: Now, gently bend the part of the chart paper where the reflected ray falls.
 
Observation:
  • When the paper is flat, both rays are visible on the same sheet.
  • When the paper is bent, the reflected ray disappears because it no longer lies in the same flat surface.
Conclusion:
This shows that the incident ray, reflected ray, and normal all lie in one plane.
If the plane is changed by bending the paper, reflection no longer follows the same path.
Laws of reflection:
  1. The angle of incidence is always equal to the angle of reflection.
  2. The incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane.
shutterstock1723884754w2570.png
Reflection of light
 
Aaron and Andy leaned closer to the mirror, watching the light glint off its surface. “So, the same rules work for all mirrors like plane, concave, or convex?” asked Andy.
“Yes,” said their father. “No matter what kind of mirror it is, light always reflects in a definite way.”
 
The laws of reflection apply to all types of mirrors, both plane and spherical. However, when several parallel rays strike a spherical mirror, an interesting effect can be observed.
 
Let us perform an activity to understand.
Activity:
To observe how parallel beams of light reflect from plane, concave, and convex mirrors.
 
download (7).png
Plane mirror producing parallel beam
 
Step 1: Set up the mirrors one by one on their stands.
Step 2: Place the stencil upright in front of the torch and fix it with a paper clip or holder.
Step 3: Switch on the torch so that light passes through the openings of the stencil, forming multiple parallel beams of light.
Step 4: Direct these parallel beams toward the plane mirror, concave mirror, and convex mirror, one after another.
Step 5: Observe how the reflected beams behave in each case.
 
Observations:
  • From the plane mirror, the reflected beams remain parallel.
  • From the concave mirror, the reflected beams come closer together, they converge.
  • From the convex mirror, the reflected beams spread out, they diverge.
Conclusion:
Even though each light ray follows the laws of reflection, the shape of the mirror affects how the reflected beams behave, a concave mirror makes them converge, a convex mirror makes them diverge, and a plane mirror reflects them parallel to each other.
Convergence:
  • When light rays come together or meet at a single point after reflection, it is called convergence.
  • A concave mirror causes parallel rays of light to converge at a point called the focus. It is also known as converging mirror.
 
concaveshutterstock82203334w600.jpg
Concave mirror
 
Divergence:
  • When light rays spread out or move away from a single point after reflection, it is called divergence.
  •  A convex mirror causes parallel rays of light to diverge after reflection. It is also known as diverging mirror.
 
convexshutterstock82203337w600.jpg
Convex mirror
 
As Andy shone a torch on the concave mirror, he noticed the light rays coming together at one point.
“Dad,” he asked, “since the concave mirror brings light together, does it make the light focus on a small area?”
“Yes,” said Dad. “A concave mirror converges light rays, concentrating them at a single point called the focus.”
 
Let us understand with an activity.
Activity:
To show that a concave mirror converges sunlight to a point.
 
Important!
Caution: This activity should be performed under adult supervision.
sunconcave4w1000.png
Concave mirror concentrating sunlight
 
Step 1: Hold the concave mirror with its reflecting surface facing the Sun.
Step 2: Allow the sunlight reflected from the mirror to fall on the sheet of paper.
Step 3: Adjust the distance between the mirror and the paper until you see a small, bright spot of light on the paper.
Step 4: Keep the mirror and the paper steady for a few minutes.
 
Observation:
After a short time, the paper begins to smoke or burn at the point where the bright spot appears.
 
Conclusion:
The concave mirror converges sunlight to a single point called the focus, where the light energy gets concentrated and produces enough heat to ignite the paper. 
Important!
Devices that use mirrors or lenses to focus sunlight onto a small area are called solar concentrators. They concentrate sunlight to heat a liquid, which produces steam. This steam can then be used to generate electricity or provide heat for various purposes, such as large-scale cooking or in solar furnaces.
In fact, solar furnaces can become so hot that they are even used to melt steel!
Next
Previous topic
Exit to the topic
Next task
Copyright © 2025 YaClass Tech Private Limited Contacts Terms and Conditions Privacy Policy