Difraction

Diffraction
Introduction

Diffraction is the phenomenon that occurs when waves bend around objects; specifically when passing through a gap and spreading out at the end. Francesco Maria Grimaldi an Italian scientist was the first to observe and identify this phenomenon.

Causes of Diffraction

Light diffraction is only caused by a single slit and therefore is not the same as young's experiment which was done with a double slit. Diffraction results in one wave of light being shifted by any diffracting object. The shift causes the wave to have interference which is destructive and constructive, the intensity of the wave increases when it is constructive interference and opposite in destructive.

Diffraction in Light

Light diffraction refers to the spreading of light that occurs when a beam of light interacts with an object. Based on the object type and the specific details, diffracted light is perceived in different ways. An example of this is when a light beam is aimed at the object edge, the light appears to bend around the object. This demonstrates that light shows wavelike properties. Diffraction does not change the wavelength as when light is made up of only one colour (monochromatic) then the bands it will produce will produce the same colour, red light produces a bigger pattern compared to blue light showing that diffraction rises with wavelength. Therefore, the pattern observed when using white light will contain bands (fringes) containing the spectrum of colours from red to violet. The amount it diffracts depends on the size of the wavelength of light passing through and the size of the gap its passing through.

http://www.bbc.co.uk/schools/gcsebitesize/science/images/ocr_phy_diffraction-wide.jpg

When waves diffract through a big gap they don’t spread out much as the light wavelength is smaller than the gap size.

http://www.bbc.co.uk/schools/gcsebitesize/science/images/ocr_phy_diffraction.jpg

Diffraction in Microscopes

The wavelength of light passing through also effects the amount of diffraction. A shorter wavelength gives less diffraction, this is significant for microscopes as diffraction causes image details to blur.The electrons in an electron microscope have a small wavelength and this can rectify the finer details as compared to light microscopes. This means we can improve resolution by using a different wavelength however we cannot eliminate diffraction because it is a natural result of the wave nature of light. Diffraction is also the reason we can get an enlarged image in a microscope.

Lens

When a lens forms an image of an object the image is actually a tiny diffraction pattern. For a lens or any circular hole, the image of a point object will be made up of a circular central peak (called the diffraction spot) surrounded by faint circular fringes. The intensity of light in the diffraction pattern of a circular opening is a complex diffraction pattern (due to the fact that a circular opening can be viewed as a slit of varying widths). If two point objects are quite close together, the images of the diffracted patterns would overlap. Moving the objects even closer together results in where we cannot make out if there is a single image or their are two images overlapping therefore a higher resolution is reached.

Optics

Diffraction in optics is an interaction between the light and the edge of pupil or what we call iris. The reason the image degrades and thus impairs perfect imagery is diffraction, this diffracted image referred to as a point image is called an Airy disk and diffraction forms an infinitesimal point on the retina even if other aberrations are present. It causes the light waves to spread transversely as they propagate. The size of the pupil is important and is a concern in diseases like Glaucoma where patients are taking Miotics (Drugs which constrict Pupil) which cause more diffraction.

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