Question

Light appears to travel in straight lines, since

• A. it is not absorbed by the atmosphere
• B. it is reflected by the atmosphere
• C. its wavelength is very small
• D. its velocity is very large

Hint:

Rectilinear propagation of light is an approximation valid only in the limit where the wavelength approaches zero ($\lambda \to 0$).
This is the fundamental assumption of geometrical (ray) optics.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the physical reason behind the rectilinear propagation of light (why light appears to travel in straight lines in everyday life).

Step 2: Detailed Explanation:

• Light is an electromagnetic wave. According to wave optics, when a wave encounters an obstacle or an aperture, it tends to bend around it; this phenomenon is known as diffraction.

• The extent of diffraction depends heavily on the ratio of the wavelength of the wave ($\lambda$) to the size of the obstacle or aperture ($d$).

• Significant diffraction or bending only occurs when the size of the obstacle is comparable to the wavelength ($\lambda \approx d$).

• The wavelengths of visible light are extremely small, ranging from approximately $400\text{ nm}$ to $700\text{ nm}$ ($4 \times 10^{-7}\text{ m}$ to $7 \times 10^{-7}\text{ m}$).

• Because everyday macroscopic objects are much larger than these wavelengths ($d \gg \lambda$), the diffraction effects of visible light are negligible under normal circumstances.

• As a result, the bending of light is imperceptible in daily life, causing light to appear to travel in straight lines.

Step 3: Final Answer:
Light appears to travel in straight lines because its wavelength is very small compared to everyday objects.