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Ray Model Of Light

In this article, the following are the objectives that will be discussed.

  • Understand what light is.
  • Understand what reflection is and how it is affected by different surfaces using the ray model of light.
  • Describe the effects and uses of reflecting surfaces.

What is Light?

Light is an important source of energy.

Image Credits: unsplash - Andrey Grinkevich

The Sun is a natural source of light, which can be converted into different forms of energy. 

How fast does light travel?

Light travels at the speed of 3 × 108 m/s in a vacuum. 

The speed of light changes as it travels through different media. 

Comparing between solids, liquids and gases, light travels the fastest in gases and the slowest in solids. 

How does light travel?

Light travels in a straight line. A light ray is a thin line of light coming from a source, which can be represented as shown below.


Light travels in a straight line
 

A bundle of light rays is called a beam of light, which can be represented as shown below:

bundle of light rays is called a beam of light

 

Reflection

What is reflection?

Reflection is the bouncing of light rays off a surface.

Mirror Reflection

Image Credits: unsplash - Elisa Photography

Characteristics of an image formed on a Plane Mirror

  1. An image formed is of the same size as the object.
  2. The image formed is upright.
  3. The image formed is laterally inverted.
  4. The image formed is virtual (not real, and cannot be projected onto a screen). 
  5. The distance between the object and the image from the mirror is the same. 

Law of Reflection

The diagram below shows the ray model of light to depict how a smooth and flat surface reflects light.

Law Of Reflection

What are the two laws of reflection?

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, reflected ray, and normal all lie on the same plane.                                                                                                                           

Drawing of the Ray Model Diagram

Drawing Of The Ray Model Diagram

Steps taken to draw the ray model diagram:

  • Measure the distance between the object to the mirror.
  • Since the distance between the object and the image from the mirror is the same, using the same distance, measure the distance of the image to the mirror. *The image should be drawn in dotted lines.
  • Using a ruler, draw a straight line from the image on the mirror to where the eye is. Do note that a dotted line, representing the light ray, should be drawn in the mirror and a normal line drawn as the light ray enters into the eye. 
  • To show that the light ray is reflected off the object, draw a straight line from the object to the light ray coming from the mirror.

Types of Reflection

What are the types of reflection?

There are two types of reflection, which are dependent on the texture of the surface.

  1. regular reflection
  2. irregular reflection                                        

 

  1. Regular Reflection

Regular reflection occurs on a smooth surface (e.g. plane mirror). The image formed is clear and undistorted.                      

Regular Reflection

When parallel light rays are reflected off the plane mirror, the reflected rays are also parallel to each other.

  1. Irregular Reflection

Irregular reflection, also known as diffused reflection, occurs on a rough surface (e.g. a plank of wood). No image will be formed on the surface.

Irregular Reflection

Types of Reflecting Surfaces

What are the different types of reflecting surfaces?

There are three types of reflecting surfaces:

  1. plane mirrors
  2. concave mirrors
  3. convex mirrors

 

  1. Plane Mirrors

Plane mirrors form images that are upright and undistorted.

monkey

Image Credits: unsplash - Andre Mouton

Another common use of the plane mirror is in the periscope, which is a tube with two plane mirrors that are set up at 45o to the horizontal.

  1. Concave Mirrors

Concave mirrors, also known as converging mirrors, have a surface that bends inwards. This causes the reflected light rays to focus on a single point. The image that is formed on a concave mirror is virtual and magnified.

Image Credits: unsplash - Bruno Pardini

 

Image Credits: unsplash - Kenny Eliason

Concave mirrors are often used in car headlights and dentist’s mirrors.

  1. Convex Mirrors

Convex mirrors, also known as diverging mirrors, have a surface that bends outwards. Convex mirrors cover a wider field of vision. 

The image that is formed on a concave mirror is virtual and magnified.

Image Credits: unsplash - Devon Janse van Rensburg

Convex mirrors are often used in the side mirrors in cars and blind corner mirrors along the roads.

Revision Questions

Here are some questions for us to look into on the objectives of this article.

Question 1:

Which of the following can light rays be represented?

  1. converging
  2. diverging
  3. parallel
  4. perpendicular

Choose the correct option 
 

  1. I and III only
  2. II and IV only
  3. I, II and III only
  4. all of the above

Solution:

(C) I, II and III only

Explanation:

A beam of light can only be represented in three ways, which are:

parallel-divergent-convergent


 

Question 2:

Which of the following reflections is formed on a smooth surface?

  1. irregular
  2. large
  3. regular
  4. small

Solution:

(C) regular

Explanation:

There are two kinds of reflection based on the surface texture: regular reflection and irregular reflection.

Regular reflection occurs on a smooth surface (e.g. plane mirror), where the image formed is clear and undistorted. while irregular reflection, also known as diffused reflection, occurs on a rough surface (e.g. a plank of wood), where no image is formed on the surface.

 

Question 3:

Which of the following correctly describes a virtual image?

  1. can be formed on a screen
  2. cannot be formed on a screen
  3. not real
  4. real

Choose the correct option

  1. I and III
  2. I and IV
  3. II and III
  4. II and IV

Solution:

(C) II and III

Explanation:

One of the characteristics of the image formed on a plane mirror is that it is virtual. A virtual image is not real and cannot be projected on a screen.

 

Question 4:

A boy stands 55 cm away from the mirror. A box was placed 25 cm behind the boy. What is the distance between the boy and the box in the mirror?

  1. 80 cm
  2. 110 cm
  3. 135 cm
  4. 160 cm

Solution:

(C) 135 cm

Explanation:

A boy stands 55 cm away from the mirror. A box was placed 25 cm behind the boy. What is the distance between the boy and the box in the mirror?

Distance between boy and image in the mirror

= 55 + 55 + 25

= 135 cm

 

Question 5:

Read the following statement about the law of reflection. Determine if the statement is correct or incorrect. 

“The angle of incidence is always similar to the angle of reflection.”

  1. correct
  2. incorrect

Solution:

(B) incorrect 

Explanation:

The angle of incidence is always equal to the angle of reflection.

 

Question 6:

Read the following statement about the law of reflection. Determine if the statement is correct or incorrect. 

“The incident ray and reflected ray lie on the same plane.”

  1. correct
  2. incorrect

Solution:

(A) correct

Explanation:

To be specific, the incident ray, reflected ray and normal all lie on the same plane.

 

Question 7:

The diagram shows a light ray hitting a hard surface.  

The diagram shows a light ray hitting a hard surface.

Determine the angle of reflection.

  1. 35°
  2. 55°
  3. 110°
  4. 124°

Solution:

(A) 35° 

Explanation:

Angle of incidence
= 90° - 55°
= 35°

Since the angle of incidence is equal to the angle of reflection, the angle of reflection will also be 35°.

 

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Test Yourself

QUESTION 1/5

Which of the following phenomenon is not due to the speed of light?

I: Lightning is observed before thunder is heard.
II. A straw inside a glass of water appears to be bent.
III. A swimming pool appears shallower than it actually is.
IV. A beam of light shining through the window lights up the room.

A. 

I

B. 

II

C. 

III

D. 

IV

Explanation

A beam of light lighting up the room does not depend on the speed of light. For (II) and (III), the phenomenon is due to speed of light being slower in water than in air. For (I), the phenomenon is due to the speed of light being faster than the speed of sound.
Ans: (IV) A beam of light shining through the window lights up the room.

QUESTION 2/5

Which of the following is not a source of light?

A. 

Sun

B. 

Torchlight

C. 

Candle

D. 

Radio

Explanation

Radio is a source of sound.
Ans: (D) Radio

QUESTION 3/5

Which of the following surface will show a regular reflection?

A. 

Cloth

B. 

Paper

C. 

Roads

D. 

Mirror

Explanation

Cloth, Paper and Road will show a diffused reflection due to irregularities on the surface.
Ans: (D) Mirror

QUESTION 4/5

Which mirror is used as a blind corner mirror?

A. 

Concave Mirror

B. 

Plane Mirror

C. 

Convex Mirror

D. 

Parabolic Mirrors

Explanation

A convex mirror has a wider view than a plane mirrors. Convex mirrors are used as blind corner mirrors on the road to help drivers view traffic around sharp corners.
Ans: (C) Convex Mirror

QUESTION 5/5

A ray of light passes through a glass prism and dispersion of light takes place. Which colour of the light will bent the most?

A. 

Red

B. 

Blue

C. 

Violet

D. 

Yellow

Explanation

After passing through a glass prism, the colours of white light is dispersed. Red light bends the least and violet bends the most.
Ans: (C) Violet

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