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Light - Rectilinear Propagation of Light

Grade 6ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light is a form of energy that produces the sensation of sight. It travels at an incredible speed of approximately c=3×108 m/sc = 3 \times 10^8 \text{ m/s} in a vacuum.

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Rectilinear Propagation of Light: This is the property of light by which it travels in a straight line in a homogeneous transparent medium.

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Ray of Light: A straight line with an arrowhead representing the direction of propagation of light. A collection of rays is called a 'Beam'.

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Types of Beams: 1. Parallel Beam (rays are parallel to each other), 2. Convergent Beam (rays meet at a point), 3. Divergent Beam (rays spread out from a point).

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Pinhole Camera: A simple device without a lens that forms a real, inverted image of an object on a screen based on the principle of rectilinear propagation.

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Shadows: A dark patch formed when an opaque object obstructs the path of light. It consists of the Umbra (the region of total darkness) and the Penumbra (the region of partial darkness surrounding the umbra).

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The size of the shadow depends on the relative distances between the source of light, the object, and the screen.

📐Formulae

c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s}

Magnification (m)=Height of image (hi)Height of object (ho)\text{Magnification } (m) = \frac{\text{Height of image } (h_i)}{\text{Height of object } (h_o)}

hiho=Distance of screen from pinhole (v)Distance of object from pinhole (u)\frac{h_i}{h_o} = \frac{\text{Distance of screen from pinhole } (v)}{\text{Distance of object from pinhole } (u)}

💡Examples

Problem 1:

An object of height ho=20 cmh_o = 20 \text{ cm} is placed at a distance of u=100 cmu = 100 \text{ cm} from a pinhole camera. If the length of the camera (distance from pinhole to screen) is v=20 cmv = 20 \text{ cm}, find the height of the image hih_i formed on the screen.

Solution:

Given: ho=20 cmh_o = 20 \text{ cm}, u=100 cmu = 100 \text{ cm}, v=20 cmv = 20 \text{ cm}. Using the formula: hiho=vu\frac{h_i}{h_o} = \frac{v}{u} hi=v×houh_i = \frac{v \times h_o}{u} hi=20 cm×20 cm100 cmh_i = \frac{20 \text{ cm} \times 20 \text{ cm}}{100 \text{ cm}} hi=400100 cm=4 cmh_i = \frac{400}{100} \text{ cm} = 4 \text{ cm}.

Explanation:

Because light travels in straight lines (rectilinear propagation), the object and its image form similar triangles with the pinhole. Thus, the ratio of their heights is equal to the ratio of their distances from the pinhole.

Problem 2:

Why are the images formed by a pinhole camera inverted?

Solution:

The images are inverted because of the rectilinear propagation of light. Light from the top of the object travels in a straight line through the pinhole and reaches the bottom of the screen, while light from the bottom of the object travels through the pinhole to reach the top of the screen.

Explanation:

Since light cannot curve around the edges of the tiny pinhole, the paths of the rays cross at the pinhole, resulting in an upside-down (inverted) image.

Rectilinear Propagation of Light - Revision Notes & Key Formulas | ICSE Class 6 Science