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 opening; light waves never do
• E. Longitudinal waves (like sound waves) are always spreading out in all direction; transverse waves (like light waves) travel only in straight lines

Answer 1

The correct option is 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

The necessary condition for diffraction is that the size of opening must be of the order of wavelength used. As wavelength of light waves is very small as compared to the size of door therefore light waves are not diffracted by door and students are not able to see him. On the other hand, wavelength of sound waves is comparable to the size of door therefore sound waves are diffracted by door and students are able to hear the teacher.