SOLAR RADIATION AND LIGHT
- The sun is
the primary source of heat to the earth and its atmosphere.
- The distance
that separates the earth from the sun is about 1,49,000, 000
- The surface
temperature of the sun is estimated between 5500oC and 6100oC.
radiation provides more than 99.9 percent of the energy that heats the
- The word
'insolation' is contraction of "incoming solar radiation".
Radiant energy from the sun that strikes the earth is called insolation.
travels with a speed of 2,97,600 km/ sec. It takes 8 minutes to 20 second.
Reach the earth
PROCESSES OF HEAT ENERGY TRANSFER
- Radiation is
the process of transmission of energy by Electro magnetic waves and is the
means by which energy emitted by the sun reaches the earth.
is the process of heat transfer
through matter by molecular activity.
In this process heat is transferred from one part of a body to
another or between two objects touching each other.
occurs through molecular movement.
is the process of the transfer of heat, through movement of a mass or
substance from one place to another.
is possible only in gases or fluids, for they alone have internal mass
- It is the
capacity of any surface to reflect the incoming radiation (light) OR it is
the ratio of incoming radiation to the outgoing radiation.
albedo value for earth is 34%.
constant is defined as the rate at which solar radiation is received
outside the earth's atmosphere on a surface perpendicular to the sun's
rays when the earth is at an average distance from the sun.
minute, the sun radiates approximately 56 x 1026 calories of
EFFECT OF LIGHT ON PLANTS
solar radiation is called as light.
- The visible
portion of the solar spectrum is the light with wave length ranging from
0.4 to 0.7 m.
- Light is
essential for the synthesis of the most important pigment i.e,
chlorophyll. The cholorphyll
absorbes the radiant energy and converts into potential energy of
intensity of light is measured by a standard unit called candle.
- The amount
of light received at a distance of one meter from a standard candle is
known as "Metre Candle or Lux".
- Very low
light intensity reduces the rate of photosynthesis and may even result in
the closing of the stomata detrimental to plants in many ways. This results in reduced plant
- Very high
light intermities are it increases the rate of respiration. It causes rapid loss of water, i.e., it
increases the transpiration rate of water from the plants resulting in
closure of stomata.
- The most
harmful effect of high intensity light is that it oxidise the cell
contents which is termed as "Solarisation".
oxidation is different from respiration and is called as 'Photooxidation".
Based on the response to light intensities the plants are classified as follows:
- Sciophytes: (Shade loving plants)
- The plants that grow better under partially
shadded (low light) conditions e.g., betevines, buckwheat, turmeric etc.,
- Heliophytes: (sun loving Plants )
- Many species
of plants produce maximum dry matter under high light intersities when the
moisture is available at the optimum level, e.g. maize, sorghum, rice
Duration of light: The response of plants to the relative length of the day and night is known as photoperiodism.
i) Long day plants:
- The plants which develop and produce
normally when the photoperiod is greater than the critical minimum
(greater than 12 hours) e.g. cereals, potato, sugar beet, wheat, barley
ii) Short day plants:
- The plants
which develop normally when the photoperiod is less than the critical
maximum (less than 12 hours) e.g. tobacco, soybean, millets, maize,
iii) Indeterminate or day neutral plants:
- Those plants which are not affected by photo
period, e.g., Tomato, Cotton, Sweet potato, pineapple etc.,
Orientation of leaves:
- The change
of position or orientation of organs of plants caused by light is usually
called as "Phototropisum".
Photomorphogenesis: Change in the morphology of plants due to light. This is mainly due to ultra violet and violet rays of the sun.
Instruments used for measuring solar radiation
1. Bellanis pyranometer
2. Sunshine recroder
3. Line quantum sensor
5. Lux meter measures the light intensity