"The person standing behind the open door inside the room can hear the voice of the person standing on the other side of the door but they cannot see each other". Give an experiment based on the diffraction for this statement.

(N/A) The condition for significant diffraction is that the wavelength of the wave $(\lambda)$ must be comparable to the size of the obstacle or aperture $(d)$.
For light, the average wavelength is $\lambda \approx 6 \times 10^{-7} \, m$. The width of a door is $d \approx 1 \, m$. The ratio $\frac{\lambda}{d} \approx 6 \times 10^{-7}$, which is extremely small. Therefore, light waves do not bend significantly around the corners of the door, making the diffraction of light negligible, and the people cannot see each other.
For sound, the frequency of human speech is typically between $100 \, Hz$ and $400 \, Hz$. Taking a frequency of $330 \, Hz$ and the speed of sound $v = 330 \, m/s$, the wavelength is $\lambda = \frac{v}{f} = \frac{330}{330} = 1 \, m$. Since the wavelength $\lambda = 1 \, m$ is comparable to the door width $d = 1 \, m$, the ratio $\frac{\lambda}{d} = 1$. This leads to significant diffraction of sound waves, allowing the sound to bend around the corners of the door and reach the person inside.