April 11 Day 20

Check your understanding
A laser beam travels through a
diffraction grating to produce a
series of bright spots on a wall.
Find the wavelength of the light.
When the wavelength is much SMALLER than the opening, no diffraction occurs
Relative size MATTERS!
When the wavelength is slightly smaller/larger or on the same order of the opening, then diffraction occurs
­try it
Diffraction PRACTICE
1. Light from a HeNe laser (λ=632.8nm) falls on a slit of unknown width. A pattern formed on a screen 1.15m away, on which the central bright band is 15mm wide. How wide is the slit d?
2. Monochromatic green light of wavelength 546nm falls on a single slit with width of 0.095mm. The slit is located 75cm from a screen. How wide will the central bright be?
Light as waves...
Insect Vision
The Biological Basis of Polarization Vision in Insects How do bees, ants, crickets, mayflies and other insects "see" the polarization of light without a polaroid filter or a dichroic crystal? In all animals, invertebrates AND vertebrates, the visual pigment rhodopsin is present in the photoreceptor membrane of the visual cells. These molecules absorb a maximum of linearly polarized light when the electric field vibrates in the direction of their dipole axis. In vertebrates the axes of rhodopsin molecules are randomly oriented. But that is not the case in the eyes of insects. The compound multifaceted eyes of insects are formed by hundreds or thousands of "simple eyes" called ommatidia. Each of these subunits has its own lens, crystalline, and several long visual cells arranged in a star pattern. The light sensitive part of the visual cells are the "microvilli": an array of tubelike membranes where the pigment rhodopsin is located. All the microvilli of the visual cells of an ommatidium point towards the center of the star, forming a light detecting waveguide: the rhabdom. The secret of polarization sensitivity is that the rhodopsin molecules are aligned preferentially parallel to the axes of the microvilli tubes. Thus, in principle, each visual cell would be maximally sensitive to light polarized parallel to its microvilli.
The different shades of a flower to show what a bee sees:
A: Lens colour picture of a flower (Gelsemium sempervirens) for human vision. B: Photograph of the flower taken through an ultraviolet transmitting lens. C: False colour image taken through an optical device simulating bee shape and colour vision. D: A filtered image that removes facets. This image is the most accurate simulation of bee colour and spatial vision that has been developed, and may hold lessons for technological advances using miniaturized optics.