Light Microscopes | OCR GCSE Biology A (Gateway) Revision Notes 2016

Introduction

• Light microscopes are essential tools for observing biological structures that are too small to be seen by the naked eye.
• They are used to view tissues, cells, and organelles by focusing light through lenses to produce a visible image.

Key Components of an Optical Microscope

• Eyepiece Lens: Used for magnification.
• Objective Lenses: Varying magnification.
• Stage: Holds the slide in place.
• Light Source: Illuminates the specimen.
• Coarse and Fine Focus: Adjust clarity of the image.
• Additional Tools: Forceps, scissors, scalpel, coverslip, slides, pipette.

Preparing Specimens

• Specimens should be carefully prepared on microscope slides to avoid damage.
• Common specimens: animal (cheek cells) and plant (onion cells).

Liquid Specimen Preparation

• Use pipette to add sample to slide.
• Cover with coverslip, press gently to remove air bubbles.
• Wear gloves to prevent contamination.

Solid Specimen Preparation

• Cut small tissue sample, prepare thin layer.
• Use chemicals if needed to preserve tissue.
• Use stain for visibility (e.g., methylene blue for cheek cells, iodine for onion cells).

Preventing Dehydration

• Add a drop of water to the specimen.

Viewing the Specimen

• Start with low power objective lens for easier identification and to prevent damage.
• If image is unclear, check specimen thickness and adjust focus.

Calibration and Measurement

• Graticule: Acts as a ruler in the field of view, calibrated using a stage micrometer.
• Calibration allows for accurate measurement of cell sizes.

Limitations of Microscopy

• Inconsistency in size appearances due to 3D cell structures.
• Limited magnification compared to other types of microscopes.
• Treatment of specimens may alter cell structure.

Units and Calculation of Magnification

• Ensure consistent units (mm, µm, nm) for calculations.
• Magnification Formula:
  • Magnification = Image Size / Actual Size
  • Use equation triangle for easy computation of variables.

Worked Example

• Example calculations provided using given dimensions and magnification factors.

Standard Form and Unit Conversion

• Standard form helps in dealing with large or small numbers.
• Conversion between units using powers of 10.

Examiner Tips

• Check units in exam questions.
• Use the equation triangle for magnification calculations to avoid errors.

Conclusion

• Light microscopes are vital for biological studies, allowing detailed observation and measurement of minute structures.