These are the reactions which take place only in the presence of light or some other radiation like visible, ultraviolet and infrared. It’s defined as a chemical reaction which is completed by the absorption of energy in the form of light. The absorption of light excites atoms and molecules by shifting some of their electrons to a higher energy level and thus makes them more reactive.
These excited states have different chemical and physical properties from the original molecules. These new chemical species change to new structures and combine with each other or other molecules by transfer of the electrons, hydrogen atoms, protons or their electronic excitation energy to other molecules. In photochemical reactions free radicals are produced which initiate the chain reactions.
Example - Photochemical reaction of HCl gas
H2(g) + Cl2(g) + hυ → HCl(g)
Characters of Photochemical Reactions
- Light is a form of energy which provides the activation energy for the reaction to take place.
- The rate of reaction does not depend on temperature but it depends on the intensity of light.
- All the radiations have a different frequency which makes them have different energies. So a particular reaction can be initiated with a particular type of radiation. For example - a photon of violet light has the highest frequency and hence the highest energy, while a photon of red light has lowest frequency and energy. So the reaction which is initiated by violet light may not be initiated by red colored radiations of visible light.
- The free energy change for photochemical reactions may not be negative.
- The photosynthesis process is the most important photochemical reaction in which almost all life on earth depends.
- Plants utilize the photon energy of sunlight and convert that into stored chemical energy by forming carbohydrates. These are sourced from atmospheric CO2 and H2O while releasing the molecular for O2 which is essential for survival.
- Some other processes like bleaching of dyes or yellowing of paper by sunlight, action of sunlight on car exhaust fumes which produce ozone, are photochemical.
- Photochemical smog is a serious problem in some big cities. Ozone protects the earth from the harmful UV rays.
- Ozone is produced in the stratosphere by a photochemical dissociation of molecular oxygen (O2) into oxygen atoms. These oxygen atoms react with molecular oxygen to produce ozone (O3).
- Photography and xerography are also based upon photochemical processes.
Some photographic terms are given below:
- Aberration- This is an optical defect in a lens causing it to form a distorted image.
- Adobe RGB- This is an Adobe system created RGB working space. It is best utilized for documents that need to be converted to CMYK.
- AF Lock- Helps in preventing auto-focus operation giving more creative control for capturing the intended image.
- Angle of View- The panning of a scene that a lens can cover. The angle of view is determined by focal length of the lens.
- Aperture- It is the opening formed by the iris blades in the lens through which the light passes through and helps in exposing the film.
- Aspherical lens- These lenses are curved but have a non-spherical surface which is used to reduce aberrations.
- Barrel Distortion- This phenomenon is observed when a wide angle image is captured resulting in curved edges of the image or with skewed angle. These errors can be corrected using image editing software.
- Camera Shake- This is the main cause of blurred images.
- CCD Charge-coupled device- It’s a type of image sensor.
- Chromatic aberration- These are the reasons for overall blur. They are a category of image errors which can be corrected with low dispersion glass.
- CMYK- The four colors that are used in the printing process are cyan, magenta, yellow, and black. These images are used to print a color separation.
- Color temperature-Color of Light- This is a kelvin (K) measured method of expressing the color content and light quality.
- Depth of Field- This shows the focus of the camera on the main subject.
- Digital Zoom- It is a kind of electronic zoom where the sensor readings help the camera to interpolate and simulate a proportionate zoom.
- Electronic flash- This requires high voltage which is obtained through batteries.
- EXIF Data - EXIF or exchangeable image file data is used to evaluate photography.
- Exposure Compensation- This is used for increasing or decreasing the exposure of an image from the automatically selected exposure by camera metering system.
- F-numbers or F-stops- These numbers provide the lens aperture size. Smaller the number, larger the aperture is.
- Film Speed/ISO/ASA-ISO (International Standards Organization) and numbers such as ISO 100 or ISO 400 etc.
- Flash synchronised speed- The modern cameras have two: a top and a bottom shutter panels.
- Focal Length- The distance measured from the film to the optical center of the lens when the lens is kept focused at infinity. It is marked in mm.
- Focal Plane Shutter- This camera part exposes the image by moving a couple of light-blocking screens across the image sensor.
- High Contrast - It’s a wide range of density in a print or negative.
- Highlights- This term shows the brightest areas of a subject, a negative, a print or a slide.
- Hot Shoe- The term is given for flash-to-camera contact and eliminates the need for a PC cord.
- Hyper focal Distance- This is considered as the distance measured for the nearest object while the lens is focused at infinity.
- Kelvin- The visible spectrum of light and is ascribed in terms of temperature of the color and is measured in degrees Kelvin (K).
- Lens and Lens speed- A type of optical glass used to collect and transfer the rays of light and form a sharp image on the film, paper or on a projection screen. Lens speed could be 400 mm lens with a maximum aperture of F/3.5 (fast) or 28mm F/3.5 lens (slow).
- Optical Zoom- It is a true zoom which produces the best photo quality.
- Over exposure - This results due to improper exposure and causes over exposed images to look too light.
- Pict Bridge- This is a standardized technology in which computer is not necessary.
- Panning- This helps in adding motion to a photograph.
- Perspective- Representation of a three-dimensional image in a two-dimensional format.
- Prime Lens- They have one focal length like lens with focal length 135 mm is a prime lens.
- RGB- A color model additive in nature that starts with black and in the absence of light.
- Saturation- It is the intensity of a color.
- Shutter Lag- This shows the delayed time difference of pressing the shutter-release button and the photo actually taken. This might vary from one digital camera to another.
- Shutter Speed- The length of time that aperture is open to capture the light medium. For example, 1/30 is nothing but 1/30 of an second.
- Single-Lens-Reflex (SLR) Camera- The functions of these types of cameras include the flash control and light metering etc.
- Under-exposure- This condition shows too little light reaching the film or camera sensor.
- Unipod - This is also known as monopod and is a single-legged support used to hold the camera steady.
- Vignetting- This is a kind of a failing brightness at image edges, and could be observed in a slide or print due to a poorly designed lens, improper lens hood or too many filters.
- Wide-Angle Lens- A kind of lens with shorter focal length accompanied with wider field view.
- Zoom Lens- These kind of lenses are used to adjust wide view range with respect to the focal length.
- Photographic Solution- Many chemicals are used in black and white photographic processing. Developing of film is generally done in closed canisters. Tray processing with baths where photographs are developed, stop baths, fixing baths, and rinse steps are used in print processing. Other treatments include use of hardeners, intensifiers, reducers, toners, and hypo eliminators. Developer solutions and powders are often highly alkaline glacial acetic acid which are used in making the stop bath. Stop baths are usually weak solutions of acetic acid. Some common intensifiers are hydrochloric acid and potassium dichromate or potassium chloro chromate. The sulphide tonner or silver with gold, selenium, uranium, platinum or iron solutions are used in toning a print.
This is a very important step in photography. This process can be done only with the exposed film. The exposed film is developed only in the dark using some alkaline solutions like quinol or pyrogallol. The alkaline solutions act as developers by reducing silver bromide to silver. This formation of silver depends on the intensity of light during the exposure time. After the developing is finished, the image is visible and this developed picture is called a negative. In this picture it is observed that the brighter parts of the picture appear darker.
This is the next step in photography. During the developing some silver bromide is unreactive and that will be removed in this process. The removal of unchanged silver bromide is called fixing. The film is immersed in sodium thio-sulphate solution to remove AgBr as a complex ion. After fixing, the film is ready to be printed out.
This is the process by which the actual photograph of the object is obtained. Printing is done on printing paper which is coated with silver salt. The fixed plate is kept on the paper and exposed to light. The negative is obtained on printing out the paper. Thus the actual photograph of the object is obtained.