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Light: Shadows and Reflections - Does Light Travel in a Straight Line?

Grade 7CBSE

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

🔑Concepts

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Rectilinear Propagation of Light: This is the property of light that describes its tendency to travel in a straight line in a transparent medium.

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A 'Ray' of light is a straight line with an arrow head showing the direction of propagation, denoted conceptually as a path R⃗\vec{R}.

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Shadow Formation: Shadows are formed because light travels in straight lines and cannot bend around opaque objects. If light could bend, there would be no dark regions behind objects.

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Pinhole Camera: This device works on the principle of rectilinear propagation. Light from the top of an object travels in a straight line through the pinhole to the bottom of the screen, creating an inverted image.

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Evidence: The most common experiment involves looking at a candle through a straight pipe versus a bent pipe; the flame is only visible through the straight pipe.

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Speed of Light: Light travels at an incredibly high speed of approximately 3×108 m/s3 \times 10^8 \text{ m/s} in a vacuum.

📐Formulae

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

Magnification (Pinhole Camera)=Height of ImageHeight of Object=Distance of Image from PinholeDistance of Object from Pinhole\text{Magnification (Pinhole Camera)} = \frac{\text{Height of Image}}{\text{Height of Object}} = \frac{\text{Distance of Image from Pinhole}}{\text{Distance of Object from Pinhole}}

💡Examples

Problem 1:

An object of height 10 cm10 \text{ cm} is placed in front of a pinhole camera. If the image formed on the screen is 2 cm2 \text{ cm} high, what is the magnification?

Solution:

M=2 cm10 cm=0.2M = \frac{2 \text{ cm}}{10 \text{ cm}} = 0.2

Explanation:

Magnification is the ratio of the image height to the object height. Since light travels in straight lines through the pinhole, the ratio of heights is proportional to the ratio of distances.

Problem 2:

Calculate the distance light travels in 33 seconds if its speed is 300,000 km/s300,000 \text{ km/s}. Show the multiplication.

Solution:

300000×3900000\begin{array}{r} 300000 \\ \times 3 \\ \hline 900000 \end{array} Distance = 900,000 km900,000 \text{ km}.

Explanation:

Distance is calculated by multiplying the speed of light by the time taken. This illustrates the massive distance light covers while moving in a straight path.

Problem 3:

Why is the image in a pinhole camera inverted (upside down)?

Solution:

Because light travels in a straight line (Rectilinear PropagationRectilinear \ Propagation), the ray from the top of the object passes through the pinhole and hits the bottom of the screen, while the ray from the bottom hits the top of the screen.

Explanation:

If light could curve, it could reach the same relative position on the screen, but its straight-line path forces the rays to cross at the aperture (pinhole).