Question

A teacher stands in front of the door of a classroom, and only one row of students sitting at their desks can see the teacher. The teacher speaks from the same location, and all the students in the classroom (five rows) are able to hear. Since both the light waves and the sound waves from the teacher pass through the door, why can all students hear the teacher but not see the teacher?
$A.$ The sound waves are refracted when they pass through the door, but the light waves travel too fast to be refracted when they pass through the door.
$B.$ The particle nature of light prevents light from "bending" as it passes through the door.
$C.$ Noticeable diffraction of waves can occur when the width openings are close to the wavelengths
of the waves being diffracted. Sound waves are noticeably diffracted as they pass through the
door, but light wavelengths are way too short to be noticeably diffracted.
$D.$ Sound waves have the property of spreading out when they pass through the opening; light waves
never do.
$E.$ Longitudinal waves (like sound waves) are always spreading out in all directions; transverse waves (like light waves) travel only in straight lines.

Answer 1

Hint: For solving this type of question we must remember the definition of diffraction and the properties of light and sound waves.

Complete step-by-step solution -
The process of bending of waves around the corners of an obstacle or aperture into the region of its geometrical shadow is known as diffraction. It is caused due to shifting of one wave of light by a diffracting object which causes the wave to have interference with itself. Interference can either be constructive or destructive. In constructive interference the intensity of the wave increases.
The most important condition for diffraction is that the wavelength of light must be comparable to the size of the object. We can say that diffraction occurs only when the size of the object is less than the wavelength of light.
In our given question, the wavelength of light waves is very small as compared to the size of the door therefore light waves are not diffracted by the door and students are not able to see him. On the other hand, the wavelength of sound waves is comparable to the size of the door therefore sound waves are diffracted by the door and students are able to hear the teacher.
Hence option $C$ is the correct answer which states that noticeable diffraction of waves can occur when the width openings are close to the wavelengths of the waves being diffracted. Sound waves are noticeably diffracted as they pass through the door, but light wavelengths are way too short to be noticeably diffracted.

Note: Diffraction is one of the basic properties that we must remember while solving these types of questions. Also, we must remember the property of light and sound waves, and the most important thing that light travels faster than sound waves.