Understanding Leaf Color Perception
Objects appear a certain color because of how they interact with light. Pigments within the object absorb some wavelengths of light and reflect others. The reflected wavelengths are what we perceive as the object's color.
Why Leaves Appear Green
Leaves contain chlorophyll, a pigment that primarily absorbs red and blue light wavelengths for photosynthesis. It reflects most of the green light wavelengths. This is why, in normal daylight (which contains all colors), leaves look green to our eyes.
Leaf Color Under Red Light
When a green leaf is placed under red light only:
- The red light hits the leaf surface.
- The chlorophyll pigment, which is optimized to absorb red light (and blue light), will absorb most of this incoming red light.
- Since the leaf absorbs the red light and reflects very little of it (it primarily reflects green, which is absent in the red light source), it does not send much light back to the observer's eye.
- An object that absorbs almost all incident light and reflects very little appears dark, often perceived as black or a very dark shade like black-brown.
Therefore, a green leaf observed under red light will appear black-brown.
Important Questions from Reflection and Refraction
A monochromatic plane wave is incident normally from a dielectric medium $A$ onto another dielectric medium $B$. The indices of refraction satisfy $n_A < n_B$. One-fourth of the incident energy is reflected back into medium $A$. Let $E$ be the resultant electric field due to the superposition of the incident wave and the reflected wave. Then, the ratio of the two time-averages $\langle \vec{E}^2 \rangle_{\text{min}}/\langle \vec{E}^2 \rangle_{\text{max}}$ is
- An electromagnetic wave is incident from vacuum normally on a planar surface of a non-magnetic medium. If the amplitude of the electric field of the incident wave is $E_0$ and that of the transmitted wave is $2E_0/3$, then neglecting any loss, the refractive index of the medium is
- Suppose the $yz$-plane forms a chargeless boundary between two media of permittivities $\epsilon_{\text{left}}$ and $\epsilon_{\text{right}}$ where $\epsilon_{\text{left}} : \epsilon_{\text{right}} = 1 : 2$. If the uniform electric field on the left is $\vec{E}_{\text{left}} = c(\hat{i} + \hat{j} + \hat{k})$ (where $c$ is a constant), then the electric field on the right $\vec{E}_{\text{right}}$ is
- A plane electromagnetic wave from within a dielectric medium (with $\epsilon = 4\epsilon_0$ and $\mu = \mu_0$) is incident on its boundary with air, at $z = 0$. The magnetic field in the medium is $\vec{H} = \hat{j} H_0 \cos(\omega t - kx - k\sqrt{3}z)$, where $\omega$ and $k$ are positive constants. The angles of reflection and refraction are, respectively,
A beam of unpolarized light in a medium with dielectric constant $\epsilon_1$ is reflected from a plane interface formed with another medium of dielectric constant $\epsilon_2 = 3\epsilon_1$. The two media have identical magnetic permeability. If the angle of incidence is $60^\circ$, then the reflected light
