Properties of light and reflection on plane mirror (O-level)

Lesson 1

LIGHT

İs a form of energy that enables us to see.

Sources of light

Natural sources include sun, star, glowing insects

Artificial luminous bodies include bulb, candle, kerosene lamp e.t.c.

 

An object is seen only when light from the object enters the eyes.

 

Most of the things like moon seen do not make their own light but reflect light from other sources to our eyes, such things are called nonluminous.

Some objects like sun, star, lamp etc. make their own light, these are called luminous.

 Transmission of light

Light travels from its source to another place through vacuum or a medium.

The media of light include:

• Transparent Medium is a substance that allows almost all the light to pass through it and objects are clearly seen through it. E.g. colorless water, paraffin, colorless glass.
• Translucent medium is one which allows some light to pass through it but objects are not clearly seen through it. E.g. cloudy liquid, frosted glass and oily paper.
• Opaque medium is one which does not allow light to pass through it at

 

Rays and beams

A ray is the direction of the path in which light travels.   It is represented by a straight line with an arrow on it.

A beam is a collection of rays.

There are three kinds of beams:

(i) Parallel beam. is a collection of rays which do not meet at all.

(ii) Convergent beam is a collection of rays originating from different directions and ending up at the same point.

(iii)Divergent beam is a collection of rays which originate from one point and are spread out in different directions.

Rectilinear propagation of light

 This  is the property of light travelling in a straight-line when produced from a source. It is propagated (sent outward) and it travels in straight lines

 Experiment to show that light travels in a straight line

 Experiment I

• Equal square cards A,B and D are cut from piece of thick paper board each with a hole in the middle.
• When the card are arranged as shown above with their holes in a straight line light passes through and seen on the other end.
• İf the middle card is displaced such that its hole is removed from the line, light is cut of from the other end

 

Conclusion: This shows that light travels in a straight line.

Experiment II

A candle light can be observed through a straight pipe but when the pipe is bent no light is seen

Experiment III

Formation of shadows and eclipses indicates that light travels in straight line.

 

Shadow

A shadow is formed when an object obstructs light. Shadows are formed because light travels in a straight line.

• A Shadow is formed by a point source is equally sharp as shown below

A point source is a very small source of light. İt can obtained by placing a cardboard with a small hole in front of a lamp as shown.

Note: For a point source

When the opaque object is moved near the source, the size of the shadow increases. But when the object is moved near the screen, the size of the shadow decreases.

• A shadow produced by extended source of light has a dark patch (umbra) and a lighter outer patch (penumbra) as shown below

Umbra is a region of the shadow where no light reaches at all.

Penumbra is a region of the shadow where some light reaches.

Note that for an extended source

When the opaque object is moved near the source, then the size of the umbra decreases but the size of the penumbra increases. When the object is moved near the screen, the size of the umbra increases but the size of the penumbra decreases.

 

Eclipse

An eclipse occurs when the sun, moon and earth are in a straight line. There are three types of eclipses namely: solar, annular and lunar.

Solar eclipse

Solar eclipse also called eclipse of the sun occurs when the moon is between the sun and earth such that both umbra and penumbra reaches the earth.

The area on earth covered by the umbra has total eclipse and the sun cannot be seen at all.

The area covered by penumbra has partial eclipse and only part of the sun is seen.

 

Lunar eclipse

Lunar eclipse is also called eclipse of the moon. Lunar occurs when the earth is between the sun and the moon.

During the eclipse of the moon the earth’s shadow is cast on the moon such that when the moon is at position “T,” total eclipse occurs, in position “T₁” partial eclipse occurs and when the moon is in position “T₂‘ no eclipse occurs but the moon is less brighter than usual.

Annular eclipse

Annular eclipse of the sun occurs when the sun is very far from earth and the moon is between the earth and the sun such that umbra does not reach the earth but only penumbra reaches earth.

Note: Total eclipse of the moon lasts longer than total eclipse of the sun because for total eclipse of moon, the earth which is in the middle is larger than moon in the eclipse of the sun.

 

FLOURESCENCE AND PHOSPHORESCENCE

Fluorescence is a substance, which absorbs energy and immediately releases the energy in the form of light.

Example of such a substance is Zinc sulphide. The screen of a T.V and C.R.O are made of a fluorescent substance.

Phosphorescence is a substance which absorbs the energy falling on it, stores it, and when energy stops falling on it, it releases it in the form of light.

An example of such a substance is calcium sulphide.

 

Pinhole camera

A pinhole camera works on the property of light travelling in a straight line.

A pinhole camera consists of a closed blackened box with a small hole on one face and a screen of tracing paper on the opposite face. The box of pinhole camera is blackened inside to prevent reflections inside the box.

Properties of images formed by pinhole cameras

• İt is real i.e. it can be produced on the screen. İt is formed by real rays of light reaching the screen.
• İt is inverted

Factors affecting the properties of image formed by pinhole camera

• When the distance between the object and the pin hole is reduced the image becomes bigger but less bright because the image is spread over a larger area and vice versa
• When the distance between the pin hole and screen is increase the image becomes bigger but becomes less bright because an image is spread over a bigger area. And vice versa
• When the pin hole is enlarged, the image becomes blurred but brighter because of too much light entering

 

Linear magnification

Linear magnification is the ratio of image size to object size.

Or: Linear magnification is the ratio of image distance to object distance.

So larger magnification is obtained when the object is nearer the pin-hole and smaller magnification is produced when the object is farther away.

 

Example 1.

Calculate the height of a building 150m away from a pinhole camera which produces an image 5cm high if the distance between pin hole and screen is 10cm.

Object distance = 150m

Image height = 5cm = 0.05m

Image distance = 10cm = 0.1m

Linear magnification = (Image height/object height)/(image distance /object distance)

0.05/ h)  = (150/0.05)

                                       h=75m

 

Example 2.

The length of a pin-hole camera is 25cm. An object 2m high is placed 10m from the pin-hole. Calculate the height of the image produced and its magnification.

Solution

Image distance = 25cm = 0.25m

Object height = 2m

Object distance = 10m

Linear magnification = (Image height/object height)/(image distance /object distance)

Linear magnification  = (image height/2) = (0.25/10)

= 0.025

Lens camera

• The lens focused light from the object on to film
• The diaphragm controls the amount of light reaching the film
• The shutter controls the exposure time of light reaching the film

 

Differences between a pinhole camera and a lens camera

• in a pinhole camera the image is always focused while in a lens camera the image is brought to focus by adjusting the position of the lens.
• İn pinhole camera the intensity of light entering is fixed while in a lens camera the intensity of light entering is controlled by a shutter or diaphragm.
• İn a pinhole camera the image distance is fixed while in a lens camera the image distance is not fixed.

 

Differences between the lens of a camera and eye

• The between the lens and retina is fixed while that between the lens and film in camera is variable
• The focal length of the eyes lens is variable while that of a camera is fixed
• Eye lens is living while the camera lens is artificial

 

Lesson 2

 Reflection of light

Reflection is the process by which light energy falling on a body or surface bounces off. The surface from which reflection occurs is called the reflecting surface.

 

Types of rays

Incident ray (AO) is a ray of light from source of light falling on to the reflecting surface.

Reflected ray (OB) is a ray of light which has been bounced off from the reflecting surface.

Normal (ON) is one which at 90⁰ with the reflecting surface. So this is incident and reflected along the same path

Angle of reflection (r) is the angle made by the reflected ray with the normal at the point of incidenceon the reflecting surface.

Angle of Incidence (i) is the angle made by the incident ray with the normal at of incidence on the reflecting surface.

 

The laws of reflection

First of law of reflection states that the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

Second law of reflection state that the angle of incidence is equal to angle of reflection.

 

Types of reflection

There are two types of reflection namely:

-Regular reflection.

-Irregular reflection.

 Regular reflection

Regular reflection occurs when a parallel incident beam strikes a smooth reflecting surface and a parallel reflected beam is obtained. Example of smooth plane surface is a plane mirror.

 Irregular or diffused reflection

Diffuse reflection occurs when a parallel incident beam strikes a rough surface and a scattered reflected beam is obtained.

The images formed by a plane mirror

An image in a plane mirror is formed by reflection of light as shown below

Properties of image formed by a plane mirror

 İt is virtual, e. it cannot be produced on the screen and is formed by imaginary rays.

• İt is Lateral inverted, e., the right of the object is the left of the image and the left of the object is the right of the image
• Same size as the object
• Same distance from the mirror as the object
• İt is erect

Note: The linear magnification produced by a plane mirror is 1 since image distance from the mirror is equal to object distance from the mirror. And the image size is equal to the object size.

 

Formation of image by no parallax in plane mirror

1. Obtain a sheet of paper and draw a mirror line.
2. Place the mirror on the line as shown.
3. Place the pin at least 5 cm from the mirror and obtain another pin (search pin)
4. Move the search pin behind the mirror to locate the position of the image where there is no parallax and place your second pin.

The principle of reversibility of light

States that light follows the same path if the direction of the travel of light is reversed

Experiment to demonstrate the principle of reversibility of light

1. A flash of light normal to the plane mirror surface is reflected through the same path
2. By locating the images of two pin after reflection through the plane mirror by no parallax

• Two pins P₁ and P₂ are fixed vertically in front of the plane mirror XY
• Viewing from the other side Y, pins P₃ and P₄ are placed such that they appear to be in line with the images of of P₁ and P₂ in the mirror.
• When viewed from side X, the images of P₃ and P₄ will be seen to be in line with pins P₁ and P₂
• This shows that light through pins P₁ and P₂ to the mirror then through P₃ and P₄ reverses through the same path from P₄ to P₃ to Mirror then to P₂ and P₁.

 

Formation of multiple images in thick plane mirror

Multiple  images are formed due to partial reflection and refraction at the non silvered surface of the mirror.

• İmage I₁ is formed by reflection on the glass surface P
• The image I₂ (the brightest is formed by reflection of the most light on the silvered surface Q
• Others by partial refraction

Parallax

Parallax is the apparent relative motion of two objects due to the movement of the observer.

As one travels in a car, trees, houses and other fixed objects seem to be in motion relative to one another. Parallax occurs only when the objects are at a distance from one another. For this reason parallax can be used to locate images in a plane mirror.

 

 Uses plane mirrors

• Submarine periscope

 The arrangement has two plane mirrors, each fixed at 45°. Light from the object is turned through 90° at each reflection.

 

 (ii) Pointer instruments

A plane mirror in the scale of the pointer instruments facilitates the correct reading of the value.

(iii). Optical lever

Plane mirrors are sometime attached to the galvanometer, such that light falling on the mirror as the galvanometer rotates is reflected over the scale.

(iv). Kaleidoscope

Inclined mirrors are used in Kaleidoscopes for producing different patterns of objects placed between them.

Please download Pdf: plane mirror (light for O-level)

Compiled by Dr. Bbosa Science

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